Seetha is getting impatient. Everyday she gets calls from friends and relatives asking if she has delivered yet. The due date was yesterday and her baby seems to quite comfortable inside the womb. The baby is showing no signs of wanting to come out! Seetha’s husband is anxious and wants the obstetrician to deliver the baby immediately. Is this necessary?

Waiting for the birth of a child is an exciting and anxious time. Most women give birth between 37 and 41weeks of pregnancy. Very few babies are born exactly on their due dates. It is quite normal to give birth as much as 3 weeks before or 10 days after the due date. It is important to remember that problems occur in only a small number of postterm pregnancies. Most women who give birth after the due date have no problems with the delivery and the newborn baby will do well.

The due date

The average length of pregnancy is 280 days, or 40 weeks from the first day of a woman's last menstrual period. It can be hard to predict the exact date of delivery. Only 5 per cent of babies are born on their due dates. Calculating the due date helps the obstetrician know which month of pregnancy you are in and to monitor the progress of the baby, particularly its growth. 

Women with irregular periods may require an ultrasound in early pregnancy to establish the actual due date. Even women with very regular periods may be asked to have an ultrasound scan in early pregnancy to confirm the age of a fetus and thereby, the due date. The due date should be confirmed as early in pregnancy as possible. Later, it becomes harder to set the due date accurately.

Calculating the due date

The due date is based on the assumption that the conception occurred exactly 2 weeks after the first day of the last period. The method used by all obstetricians around the world, is to add 7 days and subtract 3 months from the date of the first day of the last menstrual period.

For example, if the last menstrual period began on June 1, 2006, the due date is calculated as follows: June 1 + 7 days = June 8. June 8 minus 3 months = January 8. Therefore the estimated due date would be January 8, 2007.

Postterm or postdated pregnancy

By definition, a postterm pregnancy is one that lasts 41 weeks or longer. A pregnancy often lasts longer than expected because the exact time when the woman became pregnant is not known Usually obstetricians will wait for 1 week to 10 days beyond the calculated due date, as long as the baby’s health and the mother’s health remains normal. If a pregnancy goes beyond the due date and tests show that the fetus is being affected, labour may need to be induced. Sometimes, the obstetrician may decide on a cesarean section if tests show that the baby may not be able to tolerate labour. The most common reason is a decrease in the amniotic fluid.