The duration of normal pregnancy

The average pregnancy lasts 40 weeks, or 280 days when measured from the first day of the mother's last menstrual period (LMP). Although conception typically occurs on day 14 of the menstrual cycle, and the ‘true' average duration is therefore 266 days, by con­vention, pregnancies, even those conceived by assisted conception, where there is no LMP and the date of conception is known pre­cisely, are dated from the hypothetical LMP.

Methods for dating pregnancy

The date of the LMP and early scan measurements are both used. If the LMP is uncertain or unknown, the scan will be more accurate. Many authorities suggest that even if the LMP and the menstrual cycle length are known, ultrasound assessment of the crown-rump length before 14 weeks, or the head circumference before 20 weeks, is more accurate. This may well be correct but is almost impossible to prove in modern practice; labour induction based on the agreed due date makes the agreed due date a self-fulfilling prophecy.

Generally, scan dates which are less than menstrual dates are cor­rect because ovulation and conception late in the menstrual cycle is common. Scan dates which are earlier than the menstrual dates by a month or more are also likely to be correct. The issue of scan dates which appear to be 1 or 2 weeks further on than a certain menstrual date is tricky. Ovulation and conception during the LMP or in the first half of the cycle is rare, so it may often be correct to choose the menstrual date as the agreed date in such scenarios.

Variation in the duration of normal pregnancy

The length of gestation is skewed towards preterm delivery. Term is typically regarded as 37⁺⁰-42⁺⁰ and 80% of uninterfered with preg­nancies will deliver between these two dates. However, few pregnan­cies naturally go beyond 43 weeks, while there is a long tail of natural preterm birth right down to 24 weeks. Prolonged pregnancy is de­fined as a pregnancy that has progressed beyond 42+o weeks. The terms prolonged pregnancy, postdates, and post- term pregnancy are used interchangeably.

Factors causing variation in the length of pregnancy

There is disputed evidence about the relation between parity and length of gestation (1, 2), and there have been claims that it is longer in pregnancies with a male fetus (3). However, given the problems with estimating normality in modern practice with censored data this is probably of little practical importance. Prolonged pregnancy is increased in first pregnancies and in women with a body mass index of greater than 30 kg/m².