-13 September 1980 SA MED1CAL JOURNAL

Gestational Age - Fact or Fallacy?J. DOMMISSE

449

SUMMARY

A historical review reveals that Naegele's rule was basedon anecdotal evidence. This rule should be reviewed andcalculations of true gestational age should be based onthe probable date of ovulation and concepti<>n.

S. Afr. med. J. o 58, 449 (1980).

Contemporary perinatology is fundamentally dependenton the accurate calculation of the gestational age of thefetus or neonate. Previous concepts such as 'prematurity',based on a birth weight of less than 2 500 g, have beensupplanted by the realization that 'pre-term' is a betterreflection of fetal age. Utilizing techniques such as ultra­sonography and dynamic neonatal scoring systems, peri­natologists claim to be able to 'date' the neonate towithin 7 days, and we accept a statement such as, forexample, '32 - 33 weeks by gestational age', as a reason­ably accurate assessment.

We think of the average duration of pregnancy asbeing 40 weeks, yet the origins of this calculation arelost in antiquity and we do not realize that the so-called. aeoele's rule' was based on anecdotal and historicaleVideO>nce. We still use ' Taegele's rule' to calculate theexpected date of delivery by adding 7 days and 9 monthsto the first day of the last menstrual period. However,Franz Karl Naegele (1778 - 1851),' a highly esteemedpractitioner of Heidelberg, believed that the duration ofpregnancy was 10 menstrual cycles and therefore, onaverage, 280 days. This seems to have been based onhistorical evidence rather than on large series of observa­tion~, and was surprisingly accurate for the average 28­day cycle. Naegele also gave his name to a particular de­formi~v of the pelvis, having written extensively on themecha'nisms of labour. His son, Hermann Franz JosephNaegele, wrote the first book on fetal heart auscultationusing a moduled Piorry stethoscope: Tt is interesting thatboth father and ~on died in 185l.

Edward Righy (1747 - 1821),' who incidentally firstused the term '~,ccidental haemorrhage', studied in Heidel­berg and trailsiatee! Naegele's book, An Essay on iheMechanism of Pariur.'iion (published in 1829). This classicwork probably intrc)ddced 'Taegele's rule' to Britishobstetric practice.

The belief that the menstrual or ovarian cycle con­tinued during gestation and determined the duration of

Department of Obstetrics and Gynaecology, University ofCape Town and Groot.e Scbuur Hospital, Cape Town

J. DOMMISSE, :"ol.B. CH.B., F.R.C.O.G., Senior Lecturer andPrincipal Specialist

Dale received: 12 March 1980.

pregnancy wa accepted as recently as 192 and po iblyeven later. In that year, W. A. Jolly, then Profes or ofPhysiology at the University of Cape Town, publishedan anecdotal article,' which was largely based onpatients delivered by Professor E. C. Crichton, andattempted to show that pregnancy la ted until the eleventhperiod after conception. He considered that earlier partu­rition was due to other factors, uch as abnormal, ex­cessive uterine distension. His theory was that pregnancylasted eleven times the length of the menstrual cycle,counting from the middle day of the period. This at leastrecognized the variations which occur with differing cyclelengths. Jolly justified his incursion into the field ofclinical obstetrics by stating, 'When we consider the sumtotal of inconvenience, to say the least, suffered by humanbeings on account of uncertainty in this phere, it isevident that further information is much to be desired,and if this communication results in clarifying this prob­lem in any degree such an incursion by a physiologi twillbe justified'. nfortunately, Professor Jolly's hopes werenot fulfilled and our estimate i as inaccurate today asthen.

In biblical times 'ten lunar months' i mentioned in theWisdom of Solomon," and this duration was accepted byHippocrates. Ovid al 0 records that pregnancy lasts 10lunar months: but it should be noted that in the Romancalendar after Julius Caesar a lunar month was 29 days,12 hours and a few minutes, so that the duration ofpregnancy was 295 days or 42 weeks, somewhat 'post­mature' by our standards.

Thomas Denman (1816)0 was ahead of his time incalculating 40 weeks from the time of conception, basedon accurately known cases. He therefore advised calcu­lating 42 weeks from the last menstruation, but wiselyadmonished the practitioner 'to mention some time be­yond this date to allay anxiety'!

In 1837, W. F. Montgomery published a fascinatingbook entitled, An Exposiiion of Ihe Symploms and Signs ofPregnancy.' He quotes many sources in this book, includingWilliam Harvey (1578 - 1657) who believed that preg­nancy lasted 'ten revolution of the moon'. Hi ownopinion wa that pregnancy usually lasted 9 calendarmonths or 40 weeks.

A recently a 1949, Hermann Knau s" attempted toalter the method of calculation. The concept of con­tinued ovarian rhythm a propounded by Wilfred Shaw'and others was no longer accepted and as the usual dateof ovulation, 14 days prior to menstruation, had beendetermined, Knau now calculated, on a few personalobservations, that gestation la ted 273 days from thedate of ovulation. It was therefore essential, he stated,to establi h a patient's average cycle and thereby the

450 SA MEDIESE TYDSKRTF 13 September 1980

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probable date of ovulation before calculating the dura­tion of pregnancy.

It is open to question whether today, despite ourknowledge of the endocrinal control of ovulation, weare more accurate than the ancient Romans in calculatingthe gestational age. To them the time was relatively un­important, but we base critical clinical decisions on thesupposed accuracy of gestational age, and the acceptanceof 40 weeks as 'term'.

There are a few fundamental questions which weshould attempt to answer. Why in humans as opposed toother higher mammals is there this wide variation in thelength of gestation? Were the Romans not correct incalculating 42 weeks? This is certainly not an unusualduration of pregnancy for the healthy, well-nourishedpregnant patient and is it perhaps the high incidenceof preterm labour 2mong a large percentage of the popu­lation which has 'brought down the average'. No doubtpreterm labour, common in less well-nourished patients,was prevalent in the time of Naegele, Denman, Mont­gomery and others who sought to make this elusivecalculation. In Aberdeen the incidence of preterm labour(before 37 weeks) in primigravidas in the lower socialclasses is 8,4°~ compared with only 4,3'°~ in classes 1and 2' The prevailing incidence of preterm labour wouldtherefore obviously have influenced the calculation ofthe average duration of labour.

Is there perhaps a genetic, racial, or even a geographi­cal variation? Anderson" et al. in the USA calculated theaverage duration of pregnancy among Whites to be 279days for mC'le and 279,9 days for female infants, whereasamong Blacks the figures were 274,7 and 273,3 respec­tively. Similar but lighter differences were shown be­tween Whites and Blacks by Malan et al.'· in Cape Town,and also by Barron and Vessey" between 'British' (279days) and 'Jamaicans' (275 days) in London. However,Hytten and Leitch: who deal very thoroughly with theproblems of gestational age, consider that socio-economicfactors were probably more important in the above series.

It would, I suppose, be too much to expect obstetri­cians, paediatricians and neonatologists to stop speakingof a 32-week pregnancy dating from menstruation, whichtherefore implies that it started before ovulation! Cor­r·ectly, this is a 30-week gestation or even less if thepatient had a cycle lasting longer than the average 28days - and what woman is average! We are, in fact, not

calculating 'gestational age' but 'menstrual age', which isa nonsense concept.

Similarly, early ultrasonic reports on '6 - 8-week gesta­tions' would be even more impressive and valuable ifthey were dated from the probable date of ovulationand conception.

There have been several attempts fairly recently todetermine the average duration of pregnancy from aknown date of ovulation. Stewart," in 135 well-docu­mented cases, demonstrated a range of 250 - 285 daysand Guerrero and Florez1:l found a variation of from 253to 277 days, with a mean of 265, in 1 336 women. Theseare far more relevant than many larger series still per­sistently attempting to determine average gestation fromthe last menstrual period.

The time has surely come to replace the traditional'Naegele's rule' with one which takes into account theprobable date of ovulation based on that patient's averagemenstrual cycle. Let us at least either add or subtract thenumber of days the particular patient's cycle deviates from28 to the date calculated by the old-fashioned 'Naegele'srule' .

The popular 'wheel' used to calculate gestational ageshould be redesigned to take this cycle variation intoaccount and to discard the long outdated concept ofcontinuing ovarian rhythmicity. I would be happy toassist some enterprising pharmaceutical company indesigning such a calculator, which may stimulate a re­appraisal of the true gestational age and duration ofpregnancy.

REFERENCES

I. Cutler, 1. and Viets. H. (l96~): A Short HiSTOry of Midwifery.Philadelphia: W. B. Saunders.

2. Jolly, W. A. (1928): J. Obstet. Gynaec. Brit. Emp., 35, 258.j. Wisdom of Solomon, chapl. 7 (Cambridge Bible Commentary, 1973).

London· Camhridlle University Press.4. Montgomery, W. F. (1837): An Exposirion of the Symproms and

Signs of Pregnancy. London: Sherwood, Gilbert & Piper.5. Denman, T. (1816): An IntroducciOJJ iD the Practice of Midwifery.

lO:ldon.6. Knauss, H. H. (1949): J. Obstet. Gynaec. Brit. Emp., 56, 181.7. Shaw, W. (1926): Ibid., 33, 186.8. Hytlen, F. E. and leitch, 1. (1971):- The Physiology of Human

Pregnancy. Oxford: B1ackwell.9. Anderson, N. A.. Brown. E. W. and lyon. R. A. (1943): Amer. J.

Dis. Child., 65, 523.10. Malan, A. F., Evans, A., Smit, W. B. de V. er al. (1967): S. Afr.

med. J., 41, 6}8.I I. Barren, S. L. and Vessey, M. P. (1966): Brit. J. prey. soc. Med.,

20, 127.12. Stewart, H. l. (1952): J. Amer. med. Ass., 148, 1079.13. Guerrero. R. and Florez. P. E. (1969): lancet. 2. 268.