How long pregnancy lasts varies from woman to woman. The due date toward which you have been working is based on pregnancy lasting on average 40 weeks or 280 days from the date of the last menstrual period. Ovulation occurs around 2 weeks after this, so in effect pregnancy is about 38 weeks long. This is just an average, and it is considered normal to deliver anywhere between 37 weeks and 42 weeks from LMP.
How accurate is dating by LMP and scan?
When your dates are calculated from the LMP, this presumes that fertilisation occurs 2 weeks later. In many women, an ultrasound scan may unexpectedly shift their expected date of delivery (EDD). No-one doubts that you know when your last period was. What may have happened is that in the month that you conceived, you ovulated later (or earlier) than average, which is not at all uncommon. Sometimes the difference comes when a bleed which was thought to be the last period was in fact some bleeding in early pregnancy and you are one month further on than initially expected.
Pregnancy length when dated by LMP has a median length of 281 days, and by scan 278 days. If we look at how many women end up going 2 weeks over their due date, it is 9.5% according to dating by LMP but only 1.5% when dated by scan. This suggests that the dating of pregnancy by LMP tends to overestimate how far on you are. Therefore if scan information is available, it is preferable to use this for dating a pregnancy (presuming it was carried out during the first half of pregnancy, when dating is most accurate).
The above information is taken from a study which reviewed the computerised database of about 25,000 women delivering at Queen's Medical Centre, Nottingham between 1988 and 1995.
Why would you want to induce labour?
Many studies have been done looking at the outcome of pregnancies when the baby delivered at various gestations. After 42-43 weeks, there seems to be a consensus that there is more chance of problems for the baby. One of the indicators of how well a population's babies do is a statistical measure called perinatal mortality. This is the ratio of stillbirths and early neonatal deaths per total live births and stillbirths. It is expressed 'per 1000 births'. In the UK the overall perinatal mortality rate (PNMR) is about 8 per 1000. Considering infants weighing 1.5-2.0 kg (3lb 4oz - 4lb 7oz) the PNMR of this group is about 75 per 1000.
After 42 weeks the PNMR doubles and after 43 weeks it has tripled. Many studies have looked into how we can monitor women after this time to try & pick out those that really do warrant induction, leaving the others to labour naturally. This has included surveillance techniques such as: kick charts, fetal heart monitoring, ultrasound scans to check for liquor volume, and ultrasound assessment of blood flow in the baby's cord. In summary none of these is perfect, and even with intensive monitoring, 1 baby in 1000 will die before it is born. The underlying problem here is that the placenta starts to show signs of 'ageing' as pregnancy progresses, and eventually doesn't work as well as it did previously.
Good quality research has also found that induction of labour between 41 and 42 weeks reduces the chance that you will need a caesarean section and also minimises the risk of your baby inhaling meconium around the time of the birth. Meconium aspiration syndrome can cause significant breathing problems for newborns.
These are the reasons why, in general, we recommend induction of labour around 41 weeks and a few days. Inducing labour unnecessarily before you are due can lead to problems in labour. The cervix may not be ready to dilate and earlier induction does carry an increased risk of caesarean section for both slow labour and failed induction.
What are my chances of going into labour naturally after 40 weeks?
Actually, very good! The above study in Nottingham found that of women at 40 weeks, 65% labour spontaneously within the next week. Of those at 10 days over their dates, 60% will go into labour spontaneously within the next 3 days.