BJOG: an International Journal of Obstetrics and Gynaecology January 2002, Vol. 109, pp. 21–27

Maternal use of and outcome

David M. Fergussona, L. John Horwooda, Kate Northstoneb,*, ALSPAC Study Teamb

Objective To document the prevalence of cannabis use in a large sample of British women studied during pregnancy, to determine the association between cannabis use and social and lifestyle factors and assess any independent effects on pregnancy outcome. Design Self-completed questionnaire on use of cannabis before and during pregnancy. Sample Over 12,000 women expecting singletons at 18 to 20 weeks of gestation who were enrolled in the Avon Longitudinal Study of Pregnancy and Childhood. Methods Any association with the use of cannabis before and during pregnancy with pregnancy outcome was examined, taking into account potentially confounding factors including maternal social background and other substance use during pregnancy. Main outcome measures Late fetal and perinatal death, special care admission of the newborn , birthweight, birth length and head circumference. Results Five percent of mothers reported smoking cannabis before and/or during pregnancy; they were younger, of lower parity, better educated and more likely to use alcohol, cigarettes, coffee, tea and hard drugs. Cannabis use during pregnancy was unrelated to risk of perinatal death or need for special care, but, the babies of women who used cannabis at least once per week before and throughout pregnancy were 216g lighter than those of non-users, had significantly shorter birth lengths and smaller head circumferences. After adjustment for confounding factors, the association between cannabis use and birthweight failed to be statistically significant ( P ¼ 0.056 ) and was clearly non-linear: the adjusted mean birthweights for babies of women using cannabis at least once per week before and throughout pregnancy were 90g lighter than the offspring of other women. No significant adjusted effects were seen for birth length and head circumference. Conclusions The results of this study suggest that the use of cannabis during pregnancy was not associated with increased risk of perinatal mortality or morbidity in this sample. However, frequent and regular use of cannabis throughout pregnancy may be associated with small but statistically detectable decrements in birthweight.

INTRODUCTION findings of this literature have been somewhat inconsistent. There is some evidence that the offspring of cannabis users Over the last 30 years there has been a marked increase in may have slightly lower birthweight and/or increased risk cannabis use in developed countries including Europe, Great of preterm birth, but studies are conflicting. For example, Britain, the USA and Australasia1. The increase in cannabis Gibson et al.3, in a study of 7301 Australian births found use has, in turn, raised questions about the extent to which that the offspring of relatively heavy cannabis users had a cannabis may have harmful physical or psychological ef- higher incidence of low birthweight and were also more fects2. One aspect of this concern has focused on the extent to prone to preterm delivery. They showed that the low which cannabis use during pregnancy may lead to adverse birthweight relationship was entirely due to excess preterm outcomes including , perinatal death, congenital delivery; nevertheless, they found no relationship between abnormality, preterm delivery, intrauterine growth retarda- lower consumption of cannabis (1/wk) and adverse out- tion, reduced gestational age or other neonatal problems3–9. come, and the numbers in the heavy user group were small There is a growing literature on the fetal and neonatal (n ¼ 36). In contrast, in a study of 1226 mothers from an consequences of maternal cannabis use in pregnancy. The American prenatal clinic, Zuckerman et al.4 found that the offspring of relatively heavy cannabis users (i.e. positive urine screen) had lower mean birthweight (79g), and were shorter at birth (0.52cm) than the offspring of non-users a Christchurch Health and Development Study, Christchurch when allowance was made for a number of confounding School of Medicine, Christchurch Hospital, New Zealand factors including cigarette smoking and gestation, thus b Unit of Paediatric and Perinatal Epidemiology, University implying some growth retardation. A large study compris- of Bristol, UK ing 12,424 singleton deliveries in Boston identified 880 mothers who occasionally smoked cannabis in pregnancy, * Correspondence: Mrs K. Northstone, Unit of Paediatric and Perinatal 5 Epidemiology, Division of Health, 24 Tyndall Avenue, Bristol, BS8 229 who did so at least weekly and 137 who smoked daily . 1TQ, UK. Unadjusted data indicated increased rates of low birth-

D RCOG 2002 BJOG: an International Journal of Obstetrics and Gynaecology PII: S 1470-0328(02)01020-0 www.bjog-elsevier.com 22 D.M FERGUSSON ET AL. weight (<2500g) and preterm delivery (<37 weeks), but of environmental exposure prenatally and postnatally that adjusted analyses gave odds ratios (95% confidence inter- affect the development, health and wellbeing of children14. val) for any cannabis use of 1.07 (0.87, 1.31) and 1.02 All pregnant mothers resident in the three Bristol-based (0.82, 1.27) for low birthweight and preterm delivery, health districts of Avon with expected dates of delivery respectively. No analysis was carried out to see if there between 1 April 1991 and 31 December 1992 were eligible was any reduction in mean birthweight among cannabis for enrolment. It is estimated that 85%–90% of the eligible users. A study of 3857 in Connecticut found population took part, comprising 15,541 pregnancies. Of the adjusted risk of giving birth to a low birthweight baby these 13,921 were still pregnant at 18–20 weeks of gesta- (<2500g) to be 2.6 (1.1, 6.2) among white women who tion; 13,710 had singleton pregnancies resulting in live were regular users of cannabis. They also reported an births and were eligible for this study. There were 130 almost twofold risk of preterm delivery in the same pregnancies that resulted in perinatal death, defined as fetal women6. In a group of adolescent mothers use death of 20 weeks or more and early neonatal death during the first trimester resulted in a seven day reduction (i.e. death of a live birth within seven days) of which 113 in gestation after adjusting for various confounders (includ- were singleton pregnancies. ing smoking) and use in the second trimester gave an odds Data on cannabis use were collected by self-completion ratio of 3.8 (1.2–14.0) of having a baby that was small for questionnaires posted to the mothers at 18–20 weeks of gestational age7. Fried et al.8 also reported a reduction in gestation and returned by them in reply-paid envelopes. gestation of 0.8 weeks in the offspring of mothers who used The questions were: ‘‘How often did you smoke mari- cannabis at least six times a week during pregnancy. juana/grass/cannabis/: (a) in the six months before Despite the negative associations noted above, various you conceived, (b) in the first three months of pregnancy, studies have reported no significant use (c) between three months and now’’. For each of (a), (b), on preterm labour10,11, birthweight12 or adverse intrauterine and (c), the options given were: ‘‘Every day, two to four growth10,12. A meta-analysis of 10 studies of cannabis use times a week, once a week, less than once a week, not concluded that the evidence of cannabis use affecting at all’’. birthweight was inadequate13. Associations with cannabis use and a variety of factors Although the literature on the effects of maternal can- were investigated, and these factors were considered as nabis use during pregnancy has been steadily growing, it possible confounders in multivariate analyses. These has a number of limitations which include: the use of were: relatively small samples; the failure to provide estimates of the extent of maternal cannabis use; lack of prospec-  Age of mother at 18–20 weeks of gestation (<20, 20–24, tively collected measures of cannabis use; and failure to 25–29, 30–34, 35þ). control for factors that may potentially confound the  Parity (number of previous pregnancies resulting in a live association between cannabis use and pregnancy outcomes. or stillbirth). Against this general background, this paper reports on  Cigarette smoking just before pregnancy (0, 1–4, 5–9, the relationship between cannabis use during pregnancy 10–14, 15–19, 20–24, 25–29, 30þ). and neonatal outcomes in a large sample of British single-  Cigarette smoking in the first three months of pregnancy ton pregnancies studied prospectively during pregnancy. (grouped as above). The aims of this study were: 1. to estimate the proportion of  Cigarette smoking at 18–20 weeks of pregnancy women who reported cannabis use before and during (grouped as above). pregnancy; 2. to document the social, individual and related  Alcohol consumption before pregnancy (none, <1 drink/ factors that distinguish women who used cannabis during week, 1þ drinks/week, 1–2 drinks/day, 3þ drinks/day). pregnancy from those who did not use cannabis; and 3. to  Alcohol consumption during first three months of examine the extent to which use of cannabis during pregnancy (grouped as above). pregnancy was associated with increased risks of late fetal  Alcohol consumption at the time the mother first felt the death and perinatal mortality, reduced birthweight, birth baby move (grouped as above). length and head circumference.  Number of cups of tea/week at 18–20 weeks.  Number of cups of coffee/week at 18–20 weeks.  Use of hard drugs during pregnancy (yes/no). METHODS  Maternal ethnic origins (white, non-white).  Maternal education level achieved (five groups). The Avon Longitudinal Study of Pregnancy and Child-  Maternal height and maternal weight before pregnancy. hood (ALSPAC)1 is a survey aimed at identifying features  Sex of the child.

(In Table 2 some of these measures were dichotomised 1 Further details can be found on the ALSPAC website, http:// as shown; in the covariate adjustment analysis in Table 5, www.ich.bristol.ac.uk/alspac.html. these variables were analysed as scored above). D RCOG 2002 Br J Obstet Gynaecol 109, pp. 21–27 USE OF CANNABIS AND PREGNANCY OUTCOME 23

In order to obtain good measures of birth length and Table 2. Comparison of social and other characteristics of cannabis users newborn head circumference, ALSPAC staff made daily and non-users prior and during pregnancy. visits to the two major maternity hospitals (responsible for Variables Users Non-users P* 95% of births) in the area, and measured the babies born in the previous 24 hours, wherever possible. Babies in Special Mean maternal age (years) 25.5 27.8 <0.0001 Primiparous (%) 58.3 44.1 <0.0001 Care were measured as soon as this was not clinically Mothers with A level or higher 43.4 35.9 <0.0001 constrained. Gestation was calculated (in days) using the educational qualification (%) date of the mother’s last menstrual period when the mother Non-white ethnicity (%) 2.9 1.9 >0.05 was certain of this, but for uncertain last menstrual periods Smoking during pregnancy (%) 68.6 23.5 <0.0001 and conflicts with clinical assessment the ultrasound Drinking alcohol at least once 34.6 19.4 <0.0001 per week during pregnancy (%) assessment was used. For the purposes of analysis, gesta- Mean tea consumption 22.2 19.6 <0.001 tion was rounded down to complete weeks. Deaths were during pregnancy (cups per week) notified to the study from a variety of sources, and detailed Mean coffee consumption 10.8 9.0 <0.001 information was then abstracted from clinical records and (cups per week) during pregnancy postmortem reports. Use of illicit drugs during 6.8 0.2 <0.0001 pregnancy (%) Outcome measures for all live births surviving the Mean pre-pregnant weight (kg) 58.5 61.9 <0.0001 early neonatal period were: (a) birthweight (measured Mean pre-pregnant height (cm) 164.7 164.0 <0.05 in g); (b) birth length (cm); (c) head circumference at birth (cm); (d) preterm delivery (<37 weeks); (e) admis- * P for dichotomous outcomes are based on the chi-squared tests of sion to special care baby unit. Additionally, the perinatal independence; p values for mean differences are based on the t-test. death rate was considered. In all analyses sample sizes were based on the number with data on all variables in Cannabis use before and during pregnancy the analysis. All analyses were performed using SPSS for Windows Table 1 shows the number reporting cannabis use: 1. Version 10.0. Associations between cannabis users and before pregnancy; 2. within the first trimester of pregnancy; other categorical factors were examined using c2 tests for 3. in the second trimester of pregnancy. The table shows independence; t tests were used to examine mean differ- that before pregnancy just under 5% of the sample reported ences in continuous variables. Similarly, univariate associ- cannabis use, while during pregnancy 2%–3% reported ations between cannabis use and categorical outcome cannabis use. At each time the level of daily use was less variables used c2 tests for independence, while differences than 1%. in continuous outcomes were examined using ANOVA. Table 2 shows the sample classified into those who re- Multivariate analysis was performed using general linear ported using cannabis before or during pregnancy and those modelling. who were non-users and the associations with a variety of social and other characteristics. The table shows that cannabis users were significantly younger than non-users, RESULTS of lower parity, had a higher level of education, were more likely to smoke during pregnancy, and to consume more 12,129 of the participating mothers returned the ques- alcohol, tea and coffee and more likely to use other illicit tionnaire sent to them which was concerned with cannabis drugs. In addition, women who used cannabis were slightly smoking. Ninety percent of these had been completed by 25 taller but significantly lighter than non-users. These results weeks of gestation. Of the 113 singleton pregnancies suggest that those mothers using cannabis were more likely resulting in late fetal or neonatal death, 79 mothers had to have some characteristics that put them at higher risk of given information on their use of cannabis. adverse pregnancy outcomes than non-users.

Cannabis use in pregnancy and pregnancy outcomes Table 1. Cannabis use prior to and during pregnancy. Values are given as n (%). Table 3 reports on the relationship of the extent of Frequency of use 6 months before 1st Trimester Mid-pregnancy cannabis use before and during pregnancy and each meas- pregnancy ure of pregnancy outcome. For all measures of cannabis use Daily 109 (0.9) 61 (0.5) 53 (0.4) there was a consistent absence ( P>0.10) of association 2–4 times/week 149 (1.2) 84 (0.7) 59 (0.5) between cannabis use and perinatal death, preterm delivery Once/week 49 (0.4) 34 (0.3) 39 (0.3) and admission to special care. However, in all comparisons < Once/week 278 (2.3) 132 (1.1) 99 (0.8) there was evidence to suggest a dose–response relationship Not at all 11544 (95.2) 11827 (97.4) 11890 (97.9) between birthweight, birth length and head circumference All known 12129 (100) 12138 (100) 12140 (100) and the extent of cannabis use before and during preg- D RCOG 2002 Br J Obstet Gynaecol 109, pp. 21–27 24 D.M FERGUSSON ET AL.

Table 3. Relationship between cannabis use before and during pregnancy and outcomes of current pregnancy (number of subjects in each cell are shown in parentheses).

Outcome Weekly < Weekly Non-user P*

Pre-pregnancy usage Mean birthweight (g) 3258 (306) 3356 (277) 3430 (11487) <0.0001 Mean birth length (cm) 49.9 (197) 50.3 (173) 50.6 (7117) <0.0001 Mean head circumference (cm) 34.6 (201) 34.8 (174) 34.9 (7245) 0.009 Preterm delivery (%) 6.84 (21) 5.0 (14) 4.5 (522) 0.150 Admitted to NSCU (%) 7.5 (19) 7.4 (18) 6.1 (640) 0.490 Perinatal death (%) 0.7 (1) 0.7 (2) 0.7 (76) 0.901** Usage in 1st trimester Mean birthweight (g) 3243 (178) 3258 (131) 3428 (11769) <0.0001 Mean birth length (cm) 49.8 (116) 50.2 (78) 50.6 (7298) <0.0001 Mean head circumference (cm) 34.6 (119) 34.7 (77) 34.9 (7429) 0.033 Preterm delivery (%) 5.0 (9) 6.1 (8) 4.6 (540) 0.689 Admitted to NSCU (%) 6.6 (10) 6.3 (7) 6.1 (660) 0.953 Perinatal death (%) 1.2 (1) 0.8 (1) 0.6 (77) 0.908** Usage after 1st trimester Mean birthweight (g) 3225 (151) 3246 (98) 3428 (11832) <0.0001 Mean birth length (cm) 49.8 (97) 49.8 (56) 50.6 (7342) <0.0001 Mean head circumference (cm) 34.6 (99) 34.4 (56) 34.9 (7473) 0.010 Preterm delivery (%) 4.6 (7) 5.1 (5) 4.6 (546) 0.976 Admitted to NSCU (%) 7.9 (10) 5.0 (4) 6.1 (663) 0.658 Perinatal death (%) 1.7 (1) 0 0.7 (78) 0.695**

* P values for dichotomous outcomes are based on the c2 tests of independence; P values for the birthweight, length and head circumference are based on one way analysis of variance. ** Compared with all live births. nancy. Children whose mothers used cannabis at least once had mean birthweights that were nearly 250g lower than per week had a mean birthweight that was approximately non-users and birth lengths of 1cm shorter and head 200g lower than the mean birthweight of children whose circumferences reduced by an average of 0.3cm. Between mothers did not use cannabis and a mean birth length these extremes all measurements increased with decreasing almost 1cm shorter. In all cases these associations were cannabis use. highly statistically significant ( P<0.0001). The smaller head circumference observed with weekly use of cannabis was most significant with use before pregnancy ( P ¼ 0.009). Cannabis use adjusted for confounding factors To further examine the relationship between cannabis use and birthweight, the sample was classified using the In the light of the results shown in Table 2, it could be fourfold classification (Table 4). This method classified suggested that the associations evident in Table 4 may have the sample into four mutually exclusive dose groups, from arisen because of other features of cannabis users those who used cannabis at least once per week before and (e.g. youth, low parity, high tobacco, caffeine, alcohol throughout pregnancy to those who had never used can- and illicit drug use). To take these sources of confounding nabis. The table demonstrates a consistent dose–response into account, the data were further analysed using multiple relationship between the extent of maternal cannabis use regression to estimate the dose–response relationship during pregnancy and mean birthweight, birth length and between cannabis use before and during pregnancy and head circumference. Children whose mothers smoked birth measurements adjusted for the co-variates shown in cannabis at least weekly before and during pregnancy Table 2. Table 5 shows the relationship between cannabis

Table 4. Mean measurements at birth by cannabis use before and during pregnancy. Values are given as [n], mean (SD).

Cannabis use Birthweight (g) Birth length (cm) Head circumference (cm)

At least once per week before and throughout pregnancy [129] 3214 (517.7) [84] 49.6 (2.01) [86] 34.5 (1.31) Once per week before or during pregnancy but not throughout pregnancy [183] 3296 (588.9) [117] 50.2 (2.32) [119] 34.6 (1.56) < Once per week both before or throughout pregnancy [294] 3348 (547.1) [184] 50.2 (2.30) [185] 34.8 (1.39) Non-user [11464] 3430 (536.1) [7102] 50.6 (2.12) [7230] 34.8 (1.41) P* <0.0001 <0.0001 0.026

* P value based on one-way analysis of variance. D RCOG 2002 Br J Obstet Gynaecol 109, pp. 21–27 USE OF CANNABIS AND PREGNANCY OUTCOME 25

Table 5. Estimates of effect sizes in mean measurements at birth by cannabis use before and during pregnancy after adjustment for all covariates, using non-users as the baseline.

Cannabis use (n ¼ 9552) (n ¼ 6127) (n ¼ 6233) Birthweight (g) (95% CI) Birth length (cm) (95% CI) Head circumference (cm) (95% CI)

At least once per week before 101.90 (207.60, 3.81) 0.43 (0.95, 0.09) 0.12 (0.47, 0.23) and throughout pregnancy Once per week before or during 65.72 (23.21, 154.60) 0.28 (0.16, 0.71) 0.058 (0.23, 0.35) pregnancy but not throughout pregnancy < Once per week both before and 57.95 (9.60, 125.50) 0.02 (0.35, 0.31) 0.043 (0.18, 0.26) throughout pregnancy Non-user 0.00 0.00 0.00 P 0.030 0.225 0.845 use before and during pregnancy and mean birthweight, Although reducing the size of the effect in those who used birth length and head circumference adjusted for the effects cannabis during and throughout pregnancy, the effect in of maternal education, parity, maternal ethnicity, cigarette women who used cannabis before or during pregnancy, but smoking during pregnancy, gender of the child, alcohol, tea not throughout the whole pregnancy, was significantly and coffee consumption during pregnancy, maternal height increased for both mean birthweight and birth length. The and weight and the use of hard drugs during pregnancy. The effects of cannabis use on head circumference were still not table shows that after adjustment for confounding factors significant after adjusting for gestation. there was an almost significant ( P ¼ 0.030) but non-linear association between the extent of cannabis use during pregnancy and mean birthweight. The offspring of women DISCUSSION who smoked cannabis at least once per week before and throughout pregnancy had mean birthweights that were In this paper we have used prospectively collected data approximately 102g lower than the offspring of other on cannabis use before and during pregnancy to examine the women who had never used cannabis. The clear implication extent to which cannabis use was associated with adverse of this finding is that while occasional use of cannabis pregnancy outcomes when due allowance was made for before or during pregnancy does not have detectable confounding factors. The major findings of this analysis are adverse effects on birthweight, indeed it appears to increase reviewed below. mean birthweight, although not statistically significantly, On the basis of self-reported data, around 5% of the there is evidence that regular use of cannabis during women in this study admitted using cannabis before preg- pregnancy may result in a reduction in birthweight that is nancy and a slightly smaller proportion reported using statistically independent of maternal and social back- cannabis at some time during pregnancy. These findings ground, maternal characteristics or other substance use are generally consistent with the results of other self- behaviours during pregnancy. After adjusting for the vari- reported studies that have suggested that around 5% of ous confounding factors the association found with the women admit to using cannabis in pregnancy15.Itis, extent of cannabis use and mean birth length and head however, likely that these figures may underestimate the circumference were no longer significant ( P ¼ 0.225 and true prevalence of in pregnancy. In P ¼ 0.845, respectively). particular, Zuckerman et al.4 noted that in Boston the Table 6 repeats this analysis but also adjusts for the proportion of pregnant women who smoked cannabis would effects of gestation on the three measures of birth size. be increased by 20% if the results of urine screening were

Table 6. Estimates of effect sizes in mean measurements at birth by cannabis use before and during pregnancy after adjustment for all covariates including gestation, using non-users as the baseline.

Cannabis use (n ¼ 9521) (n ¼ 6096) (n ¼ 6202) Birthweight (g) (95% CI) Birth length (cm) (95% CI) Head circumference (cm) (95% CI)

At least once per week before 84.20 (174.70, 6.40) 0.46 (0.92, 0.01) 0.14 (0.45, 0.18) and throughout pregnancy Once per week before or during 89.22 (12.98, 165.30) 0.58 (0.19, 0.97) 0.24 (0.018, 0.51) pregnancy but not throughout pregnancy < Once per week both before 58.60 (12.91, 165.32) 0.04 (0.25, 0.33) 0.067 (0.13, 0.27) and throughout pregnancy Non-user 0.00 0.00 0.00 P 0.005 0.004 0.203

D RCOG 2002 Br J Obstet Gynaecol 109, pp. 21–27 26 D.M FERGUSSON ET AL. included. If this is also true of Avon, this would increase before and throughout pregnancy. However, there was a the proportion of mothers estimated to use cannabis in this detectable decrement in birthweight for the offspring of population to approximately 6%. A urine screen carried out women who used cannabis at least once per week before by the ALSPAC study in one clinic in an inner city area of and throughout pregnancy. These were in the region Avon thought to be likely to have among the highest of 90g lighter than offspring of other women even when prevalences of drug abuse showed 8% of 200 antenatal and due allowance was made for maternal social background, postnatal samples to be positive; thus extrapolating to the maternal characteristics and other substance use behaviours rest of the area where drug misuse is less likely, a prevalence during pregnancy. These adjusted findings may suggest a of 5% to 6% seems a reasonable estimate. Similarly, the only threshold model in which occasional or irregular use of study found to report prevalence rates for cannabis use cannabis during pregnancy does not have detectable effects among pregnant women in England showed that although on fetal growth but in which the offspring of women who only 0.75% of pregnant women who attended the Homerton used cannabis as frequently as once a week may have hospital in London in 1994 were known to have a drug reduced birthweight. To place these results in perspective, problem, 8.2% of a consecutive sample of 1000 women the reduction in birthweight associated with regular can- tested positive for cannabis through urine screening16.This nabis use before and during pregnancy was approximately suggests that self-reporting alone is not an adequate measure equal to the reduction in birthweight for the offspring of drug use in a pregnant population, and the results of women who smoke between 10 to 15 cigarettes per presented here should be interpreted with caution as it is day18. However, such a threshold effect can not be ascer- likely that any underreporting of cannabis use would under- tained from the data presented here. The number of estimate the true effect size. cannabis users was relatively small and so statistical power Comparison of those using cannabis with non-users is limited. Nevertheless, further studies with sufficient suggested that as a group, cannabis users were likely to statistical power to investigate any threshold effect would be at increased risk of adverse pregnancy outcome. Spe- be warranted. cifically, these women tended to be younger, of lower While these results suggest significant associations parity and to have higher rates of cigarette smoking, between cannabis use during pregnancy and mean birth- alcohol, caffeine and other illicit drug use during preg- weight, it should be borne in mind that there is a possibility nancy. These findings imply that even if there was no effect that the apparent associations between cannabis use and of cannabis use on pregnancy outcomes, one might expect birthweight are spurious and reflect the effects of uncon- to find a higher rate of adverse pregnancy outcomes trolled confounding factors that are associated with both amongst cannabis smokers as a result of their demographic cannabis use and birthweight. While we have attempted to background and other substance use during pregnancy. control a relatively comprehensive set of such factors, the Cannabis use during pregnancy was statistically unre- possibility that the relationships could reflect uncontrolled lated to a series of adverse outcomes including perinatal confounders cannot be dismissed. One particularly import- death, preterm delivery and neonatal admission for special ant factor is smoking. Nearly 70% of the women in our care. The number of cases experiencing these outcomes sample who used cannabis during pregnancy also smoked. was small, resulting in reduced statistical power. However, While we adjusted for smoking in the multivariate analyses these findings are generally consistent with previous we were unable to perform a stratified analysis due to studies that have found an absence of statistical asso- limited numbers. In interpreting these results it is also ciation between cannabis use and antenatal or perinatal important to consider the fact that cannabis is invariably morbidity and mortality3,5,17. The apparent exception to smoked with tobacco and the effects seen may be a direct this trend was that cannabis use during pregnancy showed result of tobacco use rather than that of cannabis. It is a consistent statistically significant unadjusted dose– invariably impossible to control for the inevitable con- response relationship with birthweight and birth length. founding effect of tobacco in this study. Women who smoked cannabis at least once per week Nonetheless, the findings are generally consistent with a before and during pregnancy had children who weighed growing body of evidence that suggests that, as a group, the approximately 250g less and who were 1cm shorter than babies of cannabis users, and particularly heavy or regular the offspring of non-users. users, are more likely to be at an increased risk of reduced The unadjusted analysis suggested a generally linear birthweight. These findings suggest that it would be prudent relationship between cannabis use and mean birthweight to advise pregnant women of the evidence that cannabis use with increasing cannabis use being associated with corres- during pregnancy and particularly heavy use of cannabis ponding declines in mean birthweight. However, after may lead to reduced fetal growth. More generally, these adjusting for confounding factors, the apparently linear findings add to the growing body of evidence that suggests association between birthweight and cannabis use was lost. that to produce optimal fetal growth and to minimise risks of The results suggest that there was little difference in adverse pregnancy outcomes, pregnant women should be adjusted birthweights between the offspring of non-users encouraged to avoid all forms of substance use behaviour and women who used cannabis less than once per week during pregnancy. D RCOG 2002 Br J Obstet Gynaecol 109, pp. 21–27 USE OF CANNABIS AND PREGNANCY OUTCOME 27

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