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Key Points

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The establishment of pregnancy dates is important not only for the mother who wants to know when to expect the delivery, it is also important for her carers, e.g.

1. for the timing of investigations such as serum screening;
2. assessment of maturity, such as in threatened preterm labour;
3. to time the induction of labour for post-dates pregnancy.

Current Practice

Current method of establishing the due date is by the last menstrual period (LMP), following Nägele's rule (Reference1). Various policies are applied whereby the LMP dates are used unless they have a discrepancy to the ultrasound dates of more than 7, 10 or 14 days. In the West Midlands, all three methods are currently being applied, with the majority of units using the +/- 7 day 'rule'. The ongoing CESDI - enquiry into preterm births (Project 27/28) uses the 10 day rule; i.e. menstrual dates are used to determine maturity unless the discrepancy with the scan dates exceeds 10 days.

  • It is worth noting that none of these dating protocols were introduced on the basis of evidence.

Available Evidence

1. LMP

Reliance on LMP is fraught with various problems. Between 10 and 45 % of women do not remember accurately (Reference2). But even if this date is accurate, the assumption that ovulation or conception occurs 14 days later is unfounded; there is a wide distribution with a positive skewness (Reference3). Such delay is furthermore unpredictable: in 50% of conception cycles with ovulation later than day 18, there was no indication on the basis of foregoing menstrual history (Reference4).

2. Scan

Ultrasound scan biometry does have error but it is smaller and of a normal distribution. Studies of scan accuracy in assisted reproduction technique (ART) pregnancies, i.e. where the precise date of the beginning of pregnancy is known, showed small systematic errors with the dating formulae in most common use. The random error had a 95% confidence interval of 4-6 days (Reference5; Reference6; Reference7). Even in routine use across 23 NHS units, the confidence interval was only 8 days (Reference8).

3. Prediction of birth date

Dating by ultrasound scan alone leads to a more accurate prediction of the birth date than dating by LMP alone or with a 14, 10 or 7 day 'rule' (Reference9). Furthermore, the policy of using scan dates alone would result in fewer pregnancies considered 'post dates' than any of the policies based on LMP with or without scan (Reference10).

Unanswered Questions

The evidence about accuracy of 1st trimester dating by CRL extends only to about 11 weeks, and the evidence on accuracy of BPD and HC is from 14/15 weeks only. There are currently no studies which have looked at the error of biometry dating at 12-14 weeks. Until such evidence emerges, dates derived from a scan during this interval are best reviewed again with the result of the 18-20 week scan.


Where results of an ultrasound dating scan (before 22 weeks) are available, the EDD should be derived from ultrasound biometry alone. (Reference11; Reference12; Reference13).

A recent meeting of the West Midlands regional ultrasound group (consisting of obstetricians, radiologists and radiographers) has unanimously agreed that there is now overwhelming evidence in favour of dating by scan alone. Two hospitals have recently already adopted a policy of scan dating (Birmingham Women's Hospital and Birmingham City Hospital).

The Perinatal Institute strongly advocates a policy of scan dating, as it is likely to reduce the number of unnecessary 'post-dates' inductions.


1. Naegele FC (1836). "Lehrbuch der Beburtshilfe fur Hebammen." 3rd Edition Heidelberg(TEB Mohr).

2. Geirsson RT, Busby-Earle RM (1991). "Certain dates may not provide a reliable estimate of gestational age." Br J Obstet Gynaecol 98: 108-109.

3.Guerrero R and Florez PE (1969). "The duration of pregnancy." The Lancet aug 2: 268-269.

4. Walker EM, Lewis M, et al. (1988). "Occult biochemical pregnancy: fact or fiction?" British Journal of Obstetrics & Gynaecology 95: 659-63. Abstract

5. Persson PH and Weldner BM (1986). "Reliability of ultrasound fetometry in estimating gestational age in the second trimester." Acta Obstetrica & Gynecologica Scandinavia 65: 481-483. Abstract

6. Geirsson RT and Have G (1993). "Comparison of actual and ultrasound estimated second trimester gestational length in in-vitro fertilized pregnancies." Acta Obstet Gynecol Scand 72: 344-346. Abstract

7. Chervenak FA, Skupski DW, et al. (1998). "How accurate is fetal biometry in the assessment of fetal age?" American Journal of Obstetrics & Gynecology 178: 678-87. Abstract

8. Mul T, Mongelli M, et al. (1996). "A comparative analysis of second-trimester ultrasound dating formulae in pregnancies conceived with artificial reproductive techniques." Ultrasound in Obstetrics & Gynecology 8: 397-402. Abstract

9. Mongelli M, Wilcox M, et al. (1996). "Estimating the date of confinement: ultrasonographic biometry versus certain menstrual dates." American Journal of Obstetrics & Gynecology 174(1): 278-281. Abstract

10. Gardosi J, Vanner T, et al. (1997). "Gestational age and induction of labour for prolonged pregnancy." British Journal of Obstetrics & Gynaecology 104: 792-797. Abstract

11. Geirsson RT (1991). "Ultrasound instead of last menstrual period as the basis of gestational age assignment." Ultrasound in Obstetrics & Gynecology 1: 212-219.

12. Gardosi J (1997). "Dating of pregnancy: time to forget the last menstrual period (Editorial)." Ultrasound in Obstetrics & Gynecology 9: 367-368.

13. Gardosi J and Geirsson R (1998). "Routine ultrasound is the method of choice for dating pregnancy." British Journal of Obstetrics & Gynaecology, 105: 933 -36.

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