Pregnancy: over 40 weeks

Most pregnancies last about 40 weeks, with the majority of women going into labor a week either side of this date.

The due date estimated at the beginning of a pregnancy is simply an “educated guess” about when the baby is likely to arrive. It is therefore normal for a woman to find she goes into labor one or two weeks before or after this date.

If a woman does not go into labor by time she is 40 weeks pregnant and she is pregnant for the first time, then she will be offered what is called a “membrane sweep” at her week 40 and 41 appointments.

If she has previously been pregnant, this will be offered at the week 41 appointment only.

During a membrane sweep, the vagina is examined and the cervix stimulated to produce the hormones that induce labor. If this fails to trigger labor, the woman is given the option of arranging a date to induce labor.

If a woman does not want her labor induced and the pregnancy continues for 42 weeks or longer, then the mother and baby will be monitored regularly to ensure that both are healthy.

The woman is offered ultrasound scans and the baby’s movement and heartbeat are checked. If a doctor has any concerns, they will suggest that labor is induced.

If a pregnancy takes longer than 42 weeks, the pregnancy is referred to as post-term or overdue. If this happens, there is an increased risk of stillbirth, although the majority of babies stay perfectly healthy.

Currently, the risk of stillbirth cannot be reliably predicted and all women who do not go into labor by 42 weeks are offered induced labor.

Post-Term Pregnancy Factors

A lot of women who do not go into labor by 40 weeks are not really post-term. In many cases, the due date was simply not correctly calculated.

This can happen for a number of reasons including women forgetting the exact day that their last period began, differing length of menstrual cycles and failing to have an ultrasound scan early on enough in the pregnancy to accurately predict the due date.

In cases where a pregnancy lasts beyond 42 weeks and truly is post-term, the cause is often unknown. However, factors that increase the likelihood of an overdue pregnancy include the following:

• First pregnancy
• Previous overdue pregnancy
• Family history of overdue pregnancy
• The mother is obese
• The baby is a boy
• Rarely, the cause is related to problems with the placenta.

On some occasions, if pregnancy lasts for more than 10 days beyond the due date, a doctor will suggest induced labor.

The size of the baby could complicate delivery and the amount of amniotic fluid can begin to reduce as the baby grows, which can cause pinching of the umbilical cord as the uterus contracts or the baby moves.

A baby that is overdue is also more likely to inhale what is referred to as meconium, the first fecal stool that has formed in the baby’s gut. This can cause infection or breathing problems once the baby is born.

Inducing labour

If a woman decides to have her labor induced she may be given vaginal suppositories that will help to soften and open the cervix.

A medicine called oxytocin is used to induce contractions. If the amniotic sac is still intact, the woman’s water is broken by rupturing the membranes that contain the amniotic fluid. A catheter may also be placed in the cervix to aid dilation.

Caesarean-section (C-section)

A woman who is overdue will only need a C-section if one or more of the following apply:

• Labor cannot be induced using the methods described above
• Labor does not progress in the normal way once it has started
• Tests on the baby’s heart suggest fetal distress

Further Reading

• All Pregnancy Content
• Early Signs of Pregnancy
• Is it Safe to Exercise During Pregnancy?
• Pregnancy: 0-8 weeks
• Pregnancy: 9 – 12 weeks

Written by

Sally Robertson

Sally has a Bachelor's Degree in Biomedical Sciences (B.Sc.). She is a specialist in reviewing and summarising the latest findings across all areas of medicine covered in major, high-impact, world-leading international medical journals, international press conferences and bulletins from governmental agencies and regulatory bodies. At News-Medical, Sally generates daily news features, life science articles and interview coverage.