Introduction: Fetal biometry is used for determining gestational age and estimated date of delivery (EDD). However, the accuracy of the EDD depends on the assumed length of pregnancy included in the calculation. This study aimed at assessing the actual pregnancy length and accuracy of EDD prediction based on fetal head circumference measured at the second trimester.
Material and methods: This was a population-based observational study with the following inclusion criteria: singleton pregnancy, head circumference dating in the second trimester, spontaneous onset or induction of delivery ≥ 294 days of gestation, live birth. The EDD was set anticipating a pregnancy length of 282 days. Bias in the prediction of EDD was defined as the difference between the actual date of birth and the EDD.
Results: Head circumference measurements were available for 21 451 pregnancies. Ultrasound-dated pregnancies had a median pregnancy length of 283.03 days, corresponding to a method bias of 1.03 days (95% CI; 0.89-1.16). This bias was dependent on the head circumference at dating, ranging from -1.58 days (95% CI; -3.54 to 1.12) to 3.42 days (95% CI; 1.98-4.31). The median pregnancy length, based on the last menstrual period of women with a regular menstrual cycle (n = 12 985), was 283.15 days (95% CI; 282.91-283.31). A total of 5685 (22.9%, 95% CI; 22.4% to 23.4%) and 886 women (3.6%, 95% CI; 3.3%-3.8%) were still pregnant 7 and 14 days after the EDD, respectively.
Conclusions: Second trimester head circumference measurements can be safely used to predict EDD. A revision of the pregnancy length to 283 days will reduce the bias of EDD prediction to a level comparable with other methods.
Keywords: bias; estimated date of delivery; head circumference; pregnancy length; ultrasound.
© 2018 Nordic Federation of Societies of Obstetrics and Gynecology.