A comparison of pregnancy dating methods commonly used in South Africa: a prospective study

Pregnancy dating in the South African public healthcare setting is mainly based on clinical assessment. However, the accuracy of this approach is unknown.

To compare the accuracy of different pregnancy dating methods.

We performed a prospective comparison of 2 ultrasound policies involving consecutive low-risk women in a midwife clinic in the Metro East region, Cape Town, Western Cape. Information on the last menstrual period (LMP), the 1st symphysis-to-fundal height measurement (FH) and average gestation by ultrasonographic (US) fetal biometry was recorded. Five dating methods: LMP, FH, US and their combinations, were assessed against the actual day of delivery (ADD). The main outcome measures were: (i) the days between the ADD and estimated date of delivery in pregnancies where spontaneous labour occurred and the baby had a normal birth weight; (ii) the incidence of gestational age-related outcomes; and (iii) the influence of clinical variables on dating discrepancies.

A total of 1 342 pregnancies were analysed. The accuracy of dating was similar for certain and uncertain LMP. FH was less accurate with increasing obesity. US-based dating was most accurate (for 85% of predictions within 14 days) and similarly accurate at 20 - 24 weeks and at >24 weeks. US reduced the number of assumed pre-and post-term deliveries and, in addition, was better at detecting small-for-gestational age infants (p<0.001).

Pregnancy dating by US, including those in more advanced pregnancies than currently permitted, is recommended since all non-ultrasound-based estimations of gestational age were considerably less accurate.