310 Postgrad Med J: first published as 10.1136/pgmj.25.285.310 on 1 July 1949. Downloaded from PELVIMETRY IN OBSTETRICS By H. CECIL BULL, M.R.C.P. Pregnancy is by the laws of nature the universal midplane and outlet, the surface areas of brim and experience of women, parturition no disease, a midplane and the biparietal diameter of the foetal natural act, tending to become less natural as head. Besides this we have the positive evidence woman ascends the higher reaches of civilization. of presentation and position, extended legs in The native girl of Ceylon undertakes marriage breech presentations, deformities, etc. early while the bones are soft and the muscles In radiology, diameter measurements are be- hard, pregnancy follows and parturition is an tween bony points and therefore greater than fact interlude in the working hours. With the economic because they do not take account of soft tissues. stresses of western life marriage is later, mal- Clinically or radiologically we can forecast a formations of the skeleton more frequent, par- normal labour in a pelvis of ample dimensions, or turition less easv and regard for life keener. Even the impossible delivery when the pelvis is con- Protected by copyright. so parturition for the majoritv of western women is tracted. The important group is the intermediate a natural event taking place under the kindly eye of where there may be doubt of a living child or an elder or semi-skilled female friend. danger to the mother. Here, neither clinician nor To this in the past Medicine has stood in radiologist is certain; every art and craft of benevolent and advisory capacity, willing to help medicine should therefore be impressed to save a when required. Now, the policy is to round up life or forestall a danger. Of old, ' trial labour ' pregnant women and bring them all under medical was recommended on the argument that the care in the early stages. In this antenatal period second labour would be easier, provided the she is examined to determine her general health mother survived. Now the pendulum swings, and to forecast the probable course of labour. It perhaps overswings, in the opposite direction and is an important decision to which the doctor is where there is doubt there is Caesarean section. guided by clinical judgment and the customary The gap between normal and impossible is too obstetric measurements, crude but accessory aids wide, and one way to narrow the gap is to en- http://pmj.bmj.com/ to build up the clinical picture. courage and develop a closer harmony between The pelvis has a certain geometrical shape, so clinical and radiological investigations. has the foetal head ; the soft parts are malleable to In the first place there should be a certain a degree, expulsion being effected by muscular standardization of radiological technique, an agree- contraction provided the passage is adequate. ment to accept some methods and reject others. The important factors therefore in prognosis are- Secondly, it should be recognized that radiology introduces rigid mathematics whose value can only knowledge of the pelvic canal and clinical on September 30, 2021 by guest. judgment. be appreciated in conjunction with clinical Now we have with X-rays accurate methods of judgment. measuring the diameters of the pelvic canal and of If radiology be accepted as a real help in fore- the foetal head as well as of checking the more casting the course of labour, we must integrate the obvious difficulties-multiple pregnancy, abnormal figures produced in terms of obstetrical difficulties. presentation and lie, anencephalic monster, etc. This has been done by Allen (I947) whose table is Full advantage is not made of these methods. an excellent beginning, though wide in its limits. Radiologists the world over have worked and The following paragraphs are concerned firstly written about it and it is difficult at first sight to with the radiological measurements of the pelvis understand why X-ray pelvimetry is not used in and calculations therefrom; secondly, with the every doubtful case. Radiology calculates various types of pelvis and, finally, with the mathematically, determining the diameters of the deductions that can be made from these observa- pelvic inlet, midplane and outlet, and with approxi- tions and their relation to obstetrical difficulties. mate accuracy the posterior sagittal diameters of Radiology undertakes:- in Obstetrics 311 JUIy 1949 BULL: Pelvimetry Postgrad Med J: first published as 10.1136/pgmj.25.285.310 on 1 July 1949. Downloaded from 1. Measurements 7rab (a) PELVIC INLET 4 Anteroposterior (conjugate). where a= anteroposterior diameter of mid- Transverse. plane. Right oblique. b = transverse (bispinous). Left oblique. Area. (C) OUTLET The transverse diameter is the distance between The anteroposterior is the shortest distance be- the ischial tuberosities. tween the symphysis pubis and the sacrum. In No full anteroposterior diameter can be measured most patients the point on the sacrum is the because the protrusion of the head through the anatomical promontory, but where there is subpubic arch cannot be measured. The only sacralization of the last lumbar vertebra and in linear measurement possible is the ischio-sacral, some other subjects, the first two pieces of the i.e. from the midpoint of the transverse diameter sacrum may be straight instead of curving back- to the last piece of the sacrum. wards from the promontory; the shortest dis-. tance is then to the second piece of the sacrum. (d) SUBPUBIC ANGLE Transverse-the widest diameter of the brim. Nicholson (1938) gives 750 as the ' critical Right and left oblique-from each sacroiliac joint angle, meaning that anything less than 750 is to the opposite pubic ramus. These measure- likely to lead to obstetrical difficulty. This is in- ments show if there is asymmetry of the brim but accurate as will be shown later. are not otherwise useful in diagnosis. Because of the variable shape of the pelvis no and of absolute standards of normal can be given, but the Anterior posterior sagittal-intersection Protected by copyright. the transverse with the anteroposterior diameter, following is a list of diameter measurements and divides the latter into anterior and posterior area calculations, being an average of routine sagittal diameters. These are of interest only in cases. the 'android' pelvis of Caldwell and Moloy Inlet Average normal about (1935)- Conjugate I20 mm. The brimn area is calculated from the formula of Transverse I28 mm. Nicholson (1938) Right oblique I17 mm. Left oblique 7 mm. 7rab II Area-~~ Brim area I20 sq. cm. 4 where a conjugate. Midplane b transverse diameter of inlet. Anteroposterior I20 mm. The error is least in the round pelvis but area Transverse (interspinous) I00 mm. measurements are necessarily inaccurate in an. Posterior sagittal 43 mm. http://pmj.bmj.com/ imperfect geometric figure. This being born in Midplane area 98 sq. cm. mind, they are useful in showing the combined value of the two diameters. Outlet Transverse (intertuberous) I20 mm. (b) MIDPLANE Anteroposterior (ischiosacral) 8o mm. The midplane is the area bounded in front by Subpubic angle-8o0. the .symphysis pubis, behind by the sacrum from Perpendicular-62 mm. on September 30, 2021 by guest. promontory to the last piece excluding the coccyx, and laterally by the ischium, pubis and sacro- 2. Shape sciatic ligaments. For linear measurements of Pelves are classified by the shape of the inlet, this space we take:-- by the .roportion of the conjugate to the trans- Anteroposterior-from lower edge of symphysis verse diameter. to the lower end of the last sacral segment. (a) Round (mesatipellic), the transverse diameter Lateral-the distance between the ischial spines. is equal or nearly equal to the conjugate. Posterior sagittal-from the midpoint of the (b) Long oval (dolichopellic or anthropoid), the above lateral (bispinous diameter) to the lower end transverse is smaller than the conjugate. of the last sacral segment. (c) Flat (platypellic), the transverse is greater Midplane area. Calculation is made from the than the conjugate. anteroposterior and lateral diameters according to This relationship is expressed by the brim the formula index: 312 POST GRADUATE MEDICAL JOURNAL Yuly I949 Postgrad Med J: first published as 10.1136/pgmj.25.285.310 on 1 July 1949. Downloaded from Conjugate In other words the normal posterior sagittal Transverse measurement is 40 to 50 per cent. of the length of Brim Index the conjugate and when we find it to be less than Per Cent. 40 per cent. we are dealing with an android pelvis. Round pelvis 85 to IOO The small inaccuracy due to the slightly different Long oval pelvis over ioo plane levels of conjugate and transverse diameters Flat pelvis under 85 is not significant. A fourth, the triangular shaped pelvic inlet The brim area calculated from the conjugate where the rami tend to narrow anteriorly, has and transverse diameters is but another, not very been named the 'android' pelvis by Caldwell accurate figure expressing plane area. It would be and Moloy (I935). It may be regarded as a accurate if the pelvic inlet were a circle; in- modified anthropoid shape but is imnportant be- exactitude grows in proportion as it departs cause it leads to obstetrical difficulties. In such therefrom. cases nothing is learnt from the brim index. The anterior narrowing of the pelvis means there must Midplane be more space posteriorly if the head is to pass Once through the brim in a normal pelvis the through. For recognition of the android pelvis we head has more room to move and can rotate since use the relation of the posterior sagittal diameter the backward curve of the sacrum gives a greater of the inlet to the conjugate expressed as a anteroposterior diameter.