Post-term pregnancy is when the pregnancy lasts for 42 completed weeks or more from the onset of the last menstrual period. We consider antenatal surveillance at 41 weeks or induction of labour. We, however, recommend delivery after 42 completed weeks.
The term “postdates” is ambiguous as it is imprecise – post what dates? We prefer using a more precise phrase – post-term pregnancy. A pregnancy lasts for 293 days. We don’t term any pregnancy post-term until there are 42 completed weeks (294 days) and beyond, from the onset of the last menstrual period. It is, however, correct if we assume that ovulation took place two weeks after the start of the final menstrual flow. We also note this is only true for menstrual cycles that last 28 days. The usual menstrual cycle can, however, be between 21 and 35 days. With this variation, pregnancy can either truly reach 42 weeks because it has reached 40 weeks post conception or the gestation dates were inaccurately estimated so, the pregnancy is not advanced.
We, therefore, advise that we accurately estimate the correct, gestational age before we make a diagnosis of post-term pregnancy. We typically use the onset of the last menstrual period as the reference point. Due to the uncertainty or inconsistencies that may arise with the previous method, we recommend dating the pregnancy using ultrasonography. We typically do this during early pregnancy, up to 20 weeks. We don’t recommend estimating the gestational age in the second or third trimesters due to the wide variability of the dates obtained.
Post-term pregnancy poses a risk to both the mother and the foetus that can culminate into poor maternal and perinatal outcomes.
Due to variations in the onset of the last menstrual period – perhaps inaccuracies – a significant number of babies born to women whose pregnancies were post-term, lack features typical of a postmature newborn infant. Post-mature infants are relatively uncommon. A post-mature newborn has a dry peeling skin, prominent palm and sole creases, reduced-fat under the skin, overgrown nails, excessive scalp hair, and meconium-stained skin.
Due to the elusiveness that surrounds a post-term pregnancy, we recommend that antenatal surveillance should commence at 41 weeks, using either non-stress testing and a modified or full biophysical profile. However, when clinical evidence of a compromised foetus or reduced amniotic fluid volume (oligohydramnios) ensues, we consider immediate delivery at 41 through 42 weeks. We recommend delivery as the management plan at 42 completed weeks. We ideally induce labour and monitor labour progress until delivery. At times, we may require a Caesarean delivery.
In a nutshell, a few points deserve special attention: the importance of an accurate estimation of the gestational age to diagnose a post-term pregnancy warrants a method that is accurate when doing so – ultrasonography in early pregnancy (up to 20 weeks). When the pregnancy is between 41 and 42 weeks, we recommend antenatal surveillance, that may involve a non-stress test or a biophysical profile with an option to deliver the baby if foetal compromise or oligohydramnios is present. We recommend delivery by any means as the conditions may prevail at 42 completed weeks.
You can access more information about this topic from Williams Obstetrics and MSD Manuals.