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
2. assessment of maturity, such as in threatened preterm labour;
3. to time the induction of labour for post-dates pregnancy.
Current method of establishing the due date is by
the last menstrual period (LMP), following Nägele's
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.
Reliance on LMP is fraught with various problems.
Between 10 and 45 % of women do not remember accurately
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
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
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 (5; 6; 7).
Even in routine use across 23 NHS units, the confidence
interval was only 8 days (8).
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' (9).
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
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. (11; 12; 13).
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
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.