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When is a pregnancy considered full term and what does that mean?
A pregnancy is considered full term at 39 weeks. This means it's best not to schedule your baby's delivery before that point unless there's a medical reason to do so.
On average, pregnancies last about 40 weeks (280 days) from the first day of a woman's last menstrual period. That's how your practitioner will most likely estimate your due date.
Sometimes it's in the best interest of the mom or baby to schedule an induction or a c-section instead of waiting for labor to begin. Experts need to know the gestational age at which most babies are developed enough to be born healthy so caregivers can be confident they're not scheduling a delivery too early. Babies who reach this point are considered full term.
Caregivers previously believed that babies born between 37 and 42 weeks were equally likely to be born healthy. For that reason, experts considered pregnancies to be full term at 37 weeks.
Then in 2013 the American College of Obstetricians and Gynecologists (ACOG) and the Society for Maternal-Fetal Medicine (SMFM) reviewed the research and decided to change the definition of full term to 39 weeks.
Why is 39 weeks now considered full term?
Full term is now 39 weeks because research has shown that babies born at 39 weeks were less likely to have certain health problems than those born at 37 weeks. (Babies born at 40 weeks were not significantly healthier than those born at 39 weeks.)
One study looked at the outcomes of more than 46 million births that occurred between 37 to 41 weeks. The findings applied only to women who had no medical reason to deliver before 39 weeks.
It turns out that spending two more weeks in the womb allows a baby's brain, lungs, and liver to more fully mature. Babies born at 39 or 40 weeks are also less likely to have vision and hearing problems, more likely to be born at a healthy weight, and more likely to suck and swallow well at birth, compared with those born at 37 or 38 weeks.
What's more, your baby's gestational age can be off by as much as two weeks, even if you had an early ultrasound to date your pregnancy. If you schedule a delivery before 39 weeks and your due date is off by a week or two, your baby may be born before 37 weeks gestation.
Some caregivers scheduled delivery before 39 weeks for convenience or other non-medical reasons. Their patients may have been happy to avoid the discomfort of the last few weeks of pregnancy since they believed the baby was ready to be born anyway.
ACOG and SMFM want to make it clear that delivery should only be planned before 39 weeks when continuing the pregnancy would put the mother or baby at significant health risks.
What if I go into labor before 39 weeks?
Of course, sometimes delivery before 39 weeks can't be avoided, such as when your water breaks or contractions come early. If you go into labor between 37 and 39 weeks, your caregiver will not try to delay your labor since most babies born at this point are fine. That said, it's still best to wait until 39 weeks to schedule an induction or c-section.
In other words, the risks of delivering a few weeks early are not great, but why take any risk at all with your baby's health, if you can avoid it?
Is it best for my baby to stay in my womb as long as possible?
No. According to the research, staying in the uterus past 41 weeks is not optimal for the baby's health either. That's why ACOG and SMFM now define a full-term pregnancy as one that lasts between 39 weeks, 0 days, and 40 weeks, 6 days. Babies born during this window have the best chance of being healthy, compared with babies born earlier or later.
In order to make this clearer, the two organizations developed these new labels for the last weeks of pregnancy:
- Early term: 37 weeks, 0 days to 38 weeks, 6 days
- Full term: 39 weeks, 0 days to 40 weeks, 6 days
- Late term: 41 weeks, 0 days to 41 weeks, 6 days
- Postterm: 42 weeks, 0 days and beyond
Why is it risky for my pregnancy to go past 41 or 42 weeks?
Once you reach 41 weeks, the chances of your baby having problems starts to increase and your caregiver will likely require you to come in for testing to make sure your baby is continuing to thrive in the womb. You may have a nonstress test or biophysical profile. If the tests show that the baby is not thriving, your caregiver may decide to induce delivery.
Most caregivers will want you to deliver by 42 weeks because the risks for you and your baby increase significantly after that point. Your baby may receive less oxygen and nutrients from you as your placenta starts to degrade or the amount of amniotic fluid decreases, compressing the umbilical cord. Your baby may become too big for you to deliver vaginally and has a higher chance of stillbirth after 42 weeks.
If you don't go into labor by 42 weeks, your caregiver will likely recommend inducing the birth.