The normal length of a pregnancy is regarded as any time from 37 to 42 weeks. However, once your due date passes, your caregiver may refer to the pregnancy as being 'postdates'. Most caregivers recommend inducing labour if the pregnancy ends up being 10 to 14 days overdue.
The universal use of a '40 week' gestation was first developed by a German obstetrician called Naegele in the 1800's. He declared that a pregnancy should last 10 lunar months or 40 weeks, which is 280 days, or about 9 calendar months (although we now know that a lunar month is 29.53 solar days, meaning 295 days, which is closer to 42 weeks!)
A study done in 1990 found that the average length of a human pregnancy was about 40 weeks + 8 days for first time mothers, and 40 weeks + 3 days for mothers having their second or subsequent babies. If women were left to go into labour spontaneously (or on their own), without being induced:
- About 50% of women would have their baby between 39 and 41 weeks.
- Only about 5% of babies would be born on their actual due date.
- Up to 88% of women would have their baby between 38 and 42 weeks.
- About 6-7% of babies would be born premature (before 37 weeks).
- If not induced, a small percentage of babies would be born after 42 weeks. (Taking inductions into consideration, about 1.8 % of babies were born in the 42nd week or more in Australia in 1999).
The length of each pregnancy can vary in an individual woman. Some women have their babies at different gestations (for example, one baby at 38 weeks, the next at 40 weeks and another at 42 weeks) and there is no predictable pattern. While other women can have all their babies at around the same time (for example at 41 weeks). This may inidcate they have a predetermined 'clock' that is 'set' at how long it takes her body to 'cook' a baby. In some cases, this may correlate with the length of their mother's and/or sister's pregnancies.
A few women may have 2 babies at 41 weeks, and then a third at 37 weeks. So don't rely on them all coming at the same time, when you have had 2 previous that were similar!
Physiologically, babies who are born 'overdue' often have dry, peeling skin that looks reddened. There is often less amniotic fluid draining during the labour and the baby may be more likely to pass meconium into the waters, staining them green before birth (called meconium stained liquor). There is usually no sign of vernix cream left on their skin and their fingernails can be dry and flaking. They are often referred to as being 'over cooked'!