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ULTRASOUND N August 2009 N Volume 17 N Number 3 Fetal size and dating: charts recommended for clinical obstetric practice

Pam Loughna1, Lyn Chitty2, Tony Evans3 & Trish Chudleigh4 1Academic Division of and Gynaecology, Nottingham University Hospitals NHS Trust, 2Genetics and Fetal Medicine, Institute of Health and University College London Hospitals NHS foundation Trust, London, 3Medical Physics, University of Leeds, Leeds and 4The Rosie Hospital, Cambridge, UK

less than that expected from the last menstrual period (LMP) Introduction or the head circumference measurement. This effect of using the BPD for dating has been reported by two 1 2 The charts and tables presented here represent those groups (Hadlock et al., Altman and Chitty). recommended by BMUS for routine use. The application of In view of the inaccuracies that may result from using the BPD the recommended charts in clinical practice has not been measurement, the BMUS Fetal Measurements Working Party was addressed as dating policies and the identification of growth of the opinion that the BPD should not be used in routine clinical related problems should form part of locally derived protocols. practice for the estimation of or the appropriate- ness of fetal size in later . Charts and tables for BPD General guidance measurements are therefore not presented in this document. Dating measurements are used to confirm the postmenstrual dates (if known) or to estimate the gestational age (GA) of the when the menstrual history is unknown or unreliable. Measurements for the estimation of Normally the earliest technically satisfactory measurement will be the most accurate for dating purposes. Once the gestational age (dating) gestational age has been assigned, later measurements should be used to assess fetal size and should not normally The measurements of choice for pregnancy dating are be used to reassign gestational age. dependent as shown below (Table 1). For dating charts the known variable [crown-rump length (CRL) or head circumference (HC)] is plotted along the Crown-rump length (6–13 weeks) horizontal X axis, and the unknown variable gestational age Accurate dating of pregnancy is critical to the quality of the (GA) on the vertical Y axis. Size charts plot the GA on the X national screening programme for Down’s syndrome. Whilst it is axis and the size variable on the Y axis. The plotting of recommended practice that all pregnancies are dated by measurements on a dating chart can cause confusion to the ultrasound using crown-rump length rather than menstrual dates inexperienced operator. Since a measurement acquired to 3 (NICE Antenatal Care Guidelines), in overall terms the difference date a pregnancy is made only once, it is recommended that in gestational ages calculated by the various formulae available look-up tables are used for dating purposes in preference to makes little difference to the management of the pregnancy. charts. In view of this, only dating tables are presented here. However, a difference of one or two days gestational age Fetal size can be assessed using either look-up tables or can alter a Down’s screening result from high chance to low fetal size charts. The latter are more appropriate. For serial chance or vice versa. It is therefore essential that all measurements, charts give a visual representation of the fetal laboratories involved in any form of serum screening, and all size parameters on consecutive occasions. The of ultrasound packages used in calculating risk from nuchal measurements within the normal range can also be assessed. translucency, use the same formula for calculating gestational It is recommended that departments, which adopt the charts/ age from crown-rump length measurement. tables in this document, check that any data programmed into The perfect formula does not exist, but following detailed their ultrasound and computerised patient management sys- consultation between BMUS, the Fetal Measurements Foundation tems use the GA and size equations given here. and the NHS National Screening Programme, it has been agreed that the equation given below will be adopted by all parties and by The use of the biparietal diameter (BPD) biochemistry laboratories involved in serum screening. The BPD measurement is dependant on head shape (which The recommended equation for calculation of gestational can be quantified using the cephalic index), whilst the head age from crown rump length is circumference measurement is independent of head shape. Therefore for with a dolicocephalic head shape, the GA~8:052|(CRL|1:037)1=2z23:73 head circumference will be within expected limits, but the BPD recorded will be smaller than the normal value for a given GA. If the BPD measurement is used to date such pregnancies, A dating table containing values derived from this equation is they will incorrectly be assigned a gestational age which is provided in Appendix 1.

Technique Correspondence: Pam Loughna, Nottingham University Hospitals NHS Trust, Hucknall Road, Nottingham, NG5 1PB. Pam.Loughna@ CRL measurements can be carried out trans-abdominally or nottingham.ac.uk trans-vaginally. A midline sagittal section of the whole Ultrasound 2009;17(3):161–167 DOI: 10.1179/174313409X448543 ß British Society 2009 161 Loughna et al. Fetal size and dating

Figure 2. Estimation of fetal HC from measurements of OFD (‘outer to outer’) and BPD (‘outer to outer’).

the head, using the ellipse facility. Deriving the head circumference in this way is acceptable providing the above equation is used. As the formulae used to derive a circumference may differ between manufacturers, depart- Figure 1. Measurement of CRL at (a) 6 weeks and (b) 13 weeks. ments should ensure that their machine’s software uses the correct formula. or fetus should be obtained, ideally with the embryo or fetus GA should be estimated from HC using the following formula horizontal on the screen so that the line between crown and rump is at 90u to the ultrasound beam. Linear callipers should ~ : { : 2 be used to measure the maximum unflexed length, in which loge(GA) 0 010611HC 0 000030321HC the end points of crown and rump are clearly defined (Fig. 1a z : | {7 3z : and b). The best of three measurements should be taken. 0 43498 10 HC 1 848 In very early , care must be taken to avoid inclusion of the sac (Fig. 1a) in the measurement of CRL, as this will overestimate the gestational age. It must be remembered that flexion increases with increasing gestation. Technique In measuring a flexed fetus, the gestational age will be A cross-sectional view of the fetal head at the level of the underestimated and it may be more appropriate to use the HC ventricles should be obtained. if the fetus remains flexed at 13 weeks or more. The following landmarks should be identified and the image frozen (Fig. 2): Head circumference (13–25 completed weeks) The recommended values are those of Altman and Chitty2 N rugby football shape; using the derived option as shown in Appendix 2. This involves N centrally positioned, continuous midline echo broken at calculating the HC from measurements of the biparietal one third of its length by the cavum septum pellucidum; diameter (BPD) and the occipital-frontal diameter (OFD) using N anterior walls of the lateral ventricles centrally placed the expression around the midline; N the choroid plexus should be visible within the posterior ~ z HC p(BPD OFD)=2 horn of the in the distal hemisphere.

Note that the BPD measurement uses the ‘outer-to-outer’ Measurements of OFD and BPD should be taken from an calliper positioning as described later. image with the midline echo lying as close as possible to the Many modern ultrasound machines have the capability to horizontal plane, such that the angle of insonation of the derive the HC directly from the diameters (BPD and OFD) of ultrasound beam is 90u. To measure the OFD, the intersection of the callipers should Table 1. Measurements for estimation of gestational age be placed on the outer border of the occipital and frontal (dating). edges of the skull at the point of the midline (‘outer to outer’) across the longest part of the skull (Fig. 2). To measure the Measurement Gestational age range BPD, the intersection of the callipers should be placed on the outer border of the upper and lower parietal (‘outer to Crown-rump length (CRL) 6 to 13 weeks outer’) across the widest part of the skull (Fig. 2). Provided a Head circumference (HC) {13 to 25 completed weeks technically good image is obtained, a single measurement of Femur length* (FL) {13 to 25 completed weeks both BDP and OFD is adequate.

*If head measurements are not feasible or appropriate, estimation of Femur length (13–25 completed weeks) gestational age should be made using FL. 2 {These measurements can be used beyond the gestation indicated, but The recommended values are those of Altman and Chitty as the imprecision around the estimate will increase significantly. shown in Appendix 3.

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Figure 3. Measurement of femur length.

GA should be estimated from FL using the following formula

~ : { : | z : loge(GA) 0 034375FL 0 0037254FL loge(FL) 2 306 Figure 4. Measurement of AC using the abdominal diameters method.

The recommended equation for estimating HC from GA is

Technique HC~{109:7z15:16GA{0:002388GA3 The femur should be imaged lying as close as possible to the horizontal plane, such that the angle of insonation of the Technique ultrasound beam is 90u (Fig. 3). Care should be taken to ensure that the full length of the is visualised and the view This is as described above. is not obscured by shadowing from adjacent bony parts. Provided a technically good image is obtained, a single Abdominal circumference measurement is adequate. The recommended values are those of Chitty et al.5 using the derived option as shown in Appendix 5. The recommended equation for estimating AC from GA is Measurements for the estimation of : : : 3 fetal size AC~{85 84z11 92(GA){0 0007902(GA)

Fetal size charts are used to compare the size of a fetus (of Technique known gestational age) with reference data and to compare The fetal AC is measured on a transverse section through the the size of a fetus on two or more different occasions. This can fetal abdomen that is as close as possible to circular in shape. be performed using look up tables or charts, but as it is easier Care must be taken to identify the spine and descending to identify any deviation from normal by plotting measurements posteriorly, the in the anterior one third of the on charts, the use of charts is recommended. abdomen and the stomach bubble in the same plane (Fig. 4). The measurements of choice for the estimation of fetal size The transverse abdominal diameter (TAD) and anterior- are shown in Table 2 below. As with dating, because of the posterior abdominal diameter (APAD) are measured. To potential inaccuracies with the BPD measurement, it is measure the APAD, the callipers are placed on the outer recommended that the head circumference is used to borders of the body outline, from the posterior aspect of the evaluate fetal head size rather than the BPD. For all skin covering the spine, to the anterior abdominal wall. The parameters given below, a single measurement should be TAD is measured at 90u to the APAD, across the abdomen at used, provided it is of good technical quality and obtained the widest point. using the techniques and planes described. The data used are those for an AC derived from measure- ments of two orthogonal diameters d and d using the Head circumference 1 2 expression AC5p(d1zd2)/2. The recommended values are those of Chitty et al.4 using the derived option as shown in Appendix 4. Femur length The recommended values are those of Chitty et al.6 as shown in Appendix 6. Table 2. Measurements for estimation of fetal size. The recommended equation for estimating FL from GA is

Measurement Gestational age range FL~{32:43z3:416GA{0:0004791GA3

Head circumference (HC) 13 to 42 completed weeks Abdominal circumference (AC) 13 to 42 competed weeks Technique Femur length (FL) 13 to 42 competed weeks This is as described above.

163 Loughna et al. Fetal size and dating

Acknowledgements Appendix 1

Other contributors to this document include Dr Kevin Martin, Table 1. Crown rump length dating table. Dr Paul Chamberlain and Dr Diz Shirley. The BMUS Fetal Measurements Working Party would like to GA (weekszdays) acknowledge the helpful comments received from many members of the Society during the preparation of this CRL (mm) 50th centile 5th centile 95th centile document. 56z05z26z5 66z25z47z0 76z35z67z1 86z56z07z2 References 96z66z27z4 10 7z16z37z5 11 7z26z48z0 1. Hadlock FP, Deter RL, Carpenter RJ, Park SK. Estimating fetal 12 7z36z58z1 age: effect of head shape on BPD. Am J Roentgenol 13 7z47z08z2 z z z 1981;137:83–85. 14 7 57183 15 7z67z28z4 2. Altman DG, Chitty LS. New charts for ultrasound dating of 16 8z17z38z5 pregnancy. Ultrasound Obstet Gynecol 1997;10:174–191. 17 8z27z48z6 18 8z37z59z0 3. National Collaborating Centre for Women’s and Children’s 19 8z37z69z1 Health. Antenatal Care – routine care for the healthy pregnant 20 8z48z09z2 woman. NICE/RCOG Press 2008. 21 8z58z19z3 22 8z68z19z4 4. Chitty LS, Altman DG, Henderson A, Campbell S. Charts of fetal 23 9z08z29z5 size: 2. Head measurements. Br J Obstet Gynaecol 24 9z18z39z6 1994;101:35–43. 25 9z28z49z6 z z z 5. Chitty LS, Altman DG, Henderson A, Campbell S. Charts of fetal 26 9 385100 27 9z38z610z1 size: 3. Abdominal measurements. Br J Obstet Gynaecol 28 9z48z610z2 1994;101:125–131 29 9z59z010z3 6. Chitty LS, Altman DG, Henderson A, Campbell S. Charts of fetal 30 9z69z110z3 z z z size: 4. Femur length. Br J Obstet Gynaecol 1994;101:132–135. 31 9 692104 32 10z09z210z5 33 10z19z310z6 34 10z29z410z6 35 10z29z511z0 36 10z39z511z1 37 10z49z611z1 38 10z410z011z2 39 10z510z011z3 40 10z610z111z3 41 10z610z211z4 42 11z010z211z5 43 11z010z311z5 44 11z110z311z6 45 11z210z411z6 46 11z210z512z0 47 11z310z512z1 48 11z410z612z1 49 11z410z612z2 50 11z511z012z2 51 11z511z112z3 52 11z611z112z4 53 11z611z212z4 54 12z011z212z5 55 12z111z312z5 56 12z111z312z6 57 12z211z412z6 58 12z211z413z0 59 12z311z513z0 60 12z311z613z1 61 12z411z613z1 62 12z412z013z2 63 12z512z013z3 64 12z512z113z3 65 12z612z113z4 66 12z612z213z4 67 13z012z213z5 68 13z012z313z5 69 13z112z313z6 70 13z112z413z6 71 13z212z414z0 72 13z212z514z0 73 13z312z514z0 74 13z312z614z1 75 13z412z614z1 76 13z413z014z2 77 13z513z014z2 78 13z513z014z3 79 13z613z114z3 80 13z613z114z4

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Appendix 2 Appendix 3

Table 2. Head circumference dating table: calculated from Table 3. Femur length dating table (after Altman & Chitty).2 outer to outer BPD and OFD measurements (after Altman & Chitty).2 GA (weekszdays)

GA (weekszdays) Femur length (mm) 50th centile 5th centile 95th centile z z z Head circumference (mm) 50th centile 5th centile 95th centile 10 13 0121136 11 13z212z314z1 z z z z z z 12 13 4125144 80 12 4113135 13 13z613z014z6 85 12z611z614z1 z z z z z z 14 14 1131151 90 13 2122144 15 14z313z315z3 95 13z512z415z0 z z z z z z 16 14 5135156 100 14 1130153 17 15z014z016z1 105 14z413z315z5 z z z z z z 18 15 2142163 110 15 0136161 19 15z514z416z6 115 15z314z216z4 z z z z z z 20 16 0146171 120 15 6145170 21 16z215z117z3 125 16z215z117z3 z z z z z z 22 16 4153176 130 16 4154176 23 16z615z518z1 135 17z015z618z2 z z z z z z 24 17 2160184 140 17 3162185 25 17z416z218z6 145 17z616z519z1 z z z z z z 26 17 6164192 150 18 2171193 27 18z216z619z5 155 18z517z419z6 z z z z z z 28 18 4171200 160 19 1176202 29 18z617z420z3 165 19z318z220z5 z z z z z z 30 19 2176205 170 19 6185211 31 19z418z121z1 175 20z219z121z4 z z z z z z 32 20 0183214 180 20 5193220 33 20z218z522z0 185 21z119z622z3 z z z z z z 34 20 5191222 190 21 4202226 35 21z019z322z5 195 22z020z423z2 z z z z z z 36 21 3195231 200 22 2210235 37 21z520z132z4 205 22z521z324z2 z z z z z z 38 22 1203240 210 23 1215245 39 22z420z524z3 215 23z422z125z1 z z z z z z 40 22 6211246 220 24 0224255 41 23z221z325z2 225 24z322z626z1 z z z z z z 42 23 5216255 230 24 6232265 43 24z122z126z1 235 25z323z527z1 z z z z z z 44 24 3224264 240 25 6241275 45 24z622z627z1 245 26z224z328z2 z z z z z z 46 25 2232274 250 26 5246286 47 25z523z428z0 255 27z225z229z3 z z z z z z 48 26 1240283 260 27 5255300 49 26z424z329z0 265 28z226z130z4 z z z z z z 50 27 0245293 270 28 6264312 51 27z325z130z0 275 29z327z032z0 z z z z z z 52 27 6254303 280 30 0273324 53 28z226z031z0 285 30z427z633z3 z z z z z z 54 28 5262313 290 31 1283341 55 29z226z532z0 295 31z528z635z0 z z z z z z 56 29 5271323 300 32 3293356 57 30z127z433z0 305 33z130z036z5 z z z z z z 58 30 4280334 310 33 6303374 59 31z128z334z1 315 34z431z038z4 z z z z z z 60 31 4286344 320 35 3315394 61 32z129z235z1 62 32z429z535z5 63 33z130z136z2 64 33z430z436z6 65 34z131z037z3 66 34z431z338z0 67 35z132z038z5

165 Loughna et al. Fetal size and dating

Appendix 4 Appendix 5

Figure 5. Head circumference size chart (after Chitty et al.).4 Figure 6. Abdominal circumference size chart (after Chitty et al.).5

Table 5. Abdominal circumference size table (after Chitty Table 4. Head circumference size table (after Chitty et al.).4 et al.).5

Head circumference (mm) Abdominal circumference (mm)

GA (weeks) 50th centile 5th centile 95th centile GA (weeks) 50th centile 5th centile 95th centile

12 68.157.179.2 12 55.849.062.6 13 82.270.893.6 13 67.459.675.2 14 96.084.2 107.8 14 78.970.187.7 15 109.797.5 121.9 15 90.380.5100.1 16 123.1110.6 135.7 16 101.690.9112.4 17 136.4123.4 149.3 17 112.9101.1124.7 18 149.3136.0 162.7 18 124.1111.3136.9 19 162.0148.3 175.7 19 135.2121.5149.0 20 174.5160.4 188.6 20 146.2131.5161.0 21 186.6172.1 201.1 21 157.1141.4172.9 22 198.5183.6 213.3 22 168.0151.3184.7 23 210.0194.8 225.3 23 178.7161.0196.4 24 221.2205.6 236.9 24 189.3170.6208.0 25 232.1216.1 248.1 25 199.8180.1219.5 26 242.6226.2 259.0 26 210.2189.5230.8 27 252.7235.9 269.5 27 220.4198.8242.1 28 262.5245.3 279.6 28 230.6207.9253.2 29 271.8254.3 289.4 29 240.5216.9264.2 30 280.7262.8 298.7 30 250.4225.8275.0 31 289.2270.9 307.6 31 260.1234.5285.7 32 297.3278.6 316.0 32 269.7243.1296.3 33 304.9285.8 324.0 33 279.1251.5306.7 34 312.0292.6 331.5 34 288.4259.8317.0 35 318.7298.8 338.5 35 297.5267.9327.0 36 324.8304.6 345.0 36 306.4275.8337.0 37 330.4309.8 351.0 37 315.1283.6346.7 38 335.5314.5 356.5 38 323.7291.2356.3 39 340.0318.7 361.4 39 332.1298.6365.7 40 344.0322.3 365.8 40 340.4305.8374.9 41 347.4325.3 369.6 41 348.4312.9383.9 42 350.3327.7 372.8 42 356.2319.7392.7

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Appendix 6

Figure 7. Femur length size chart (after Chitty et al.).6

Table 6. Femur length size table (after Chitty et al.).6

Femur length (mm)

GA (weeks) 50th centile 5th centile 95th centile

12 7.74.810.6 13 10.97.913.9 14 14.111.017.2 15 17.214.020.4 16 20.317.023.6 17 23.319.926.7 18 26.322.829.7 19 29.225.632.8 20 32.128.435.7 21 34.931.138.6 22 37.633.841.5 23 40.336.444.3 24 42.938.947.0 25 45.541.449.6 26 48.043.752.2 27 50.446.054.7 28 52.748.357.1 29 55.050.459.5 30 57.152.561.7 31 59.254.563.9 32 61.256.466.0 33 63.158.268.0 34 64.959.969.9 35 66.661.571.7 36 68.263.073.4 37 69.764.475.0 38 71.165.776.5 39 72.466.977.9 40 73.668.079.1 41 74.668.980.3 42 75.669.881.3

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