Your due date is calculated to be 40 weeks from the first day of your last period. Despite this, it is common and most of the time safe to have your pregnancy last longer than 40 weeks. A term pregnancy is officially between 37 and 41 weeks and 6 days.
The placenta reaches its maximum effectiveness at about 36 weeks and then slowly becomes less effective. The amniotic fluid reaches a maximum around that time as well.
After 42 weeks gestation, your pregnancy will be called “post-term” or “postdates” and there can be an increased risk of a stillbirth. Most health care providers will therefore start fetal testing at 41 weeks to make sure your baby is doing well.
This antenatal testing consists of a non-stress test (NST) and amniotic fluid level check. The NST is done by tracing the fetal heart rate over 20-30 min and looking for certain predictive heart rate patterns. An ultrasound is used to evaluate the amniotic fluid.
Most health care providers will recommend inducing your labor so that your baby is delivered by 42 weeks gestation.