The Mystery of Post Term Babies

The Mystery of Post Term Babies

The following is a student blog post by Nicholette Lambert

For all the advances in ultrasound and prenatal testing, doctors and scientists still can’t explain one of the most fundamental parts of childbirth: what causes labor to start. This fundamental gap in our understanding of childbirth has turned the question of post term babies into a difficult issue.

The most common reason that babies go past the 40 week mark is simple incorrect dating of the pregnancy. Calculating gestational age is not always an exact science. A woman whose cycle is longer than 28 days may end up with a due date that is too early for her baby’s actual gestational age. But there are some babies that are true post-term babies, meaning they stay in the womb past 42 weeks.

The concern is whether or not this is harmful to the baby. Research shows that babies who go past 41 weeks have a higher likelihood of death than those born at 39-40 weeks. So the standard approach is to induce labor after 41 weeks (Rosenstein, Cheng, Snowden, et. al., 2012).

Women’s feelings about an induction for a post term baby can be varied. While some welcome the chance to get labor started, inductions can increase the risk of c-sections. Most mothers are concerned about the possibility of stillbirth. But induction doesn’t necessarily guarantee  a healthy baby with no further problems. Babies who are born at 41 weeks or later are at a greater risk for birth complications. But they are also at a much greater risk for neonatal intensive care unit stays and SIDS as well. In fact, SIDS is the leading cause of death for post term babies. In some cases, whatever factors that delay the onset of labor may be related to conditions that could cause other difficulties and later death for the baby (Rosenstein, Cheng, Snowden, et. al., 2012).

post term

For mothers who are dealing with the issue of a post term baby, there are a few things to keep in mind:

The baby may not actually be post term. If the mother’s menstrual cycles have been irregular or especially long, gestation calculated simply by date of last menstrual period may be very inaccurate. If the due date has been calculated with a pregnancy wheel and the mother’s cycle does not follow the standard 28 days with ovulation mid-cycle, the baby is probably not post term and an induction could potentially cause harm.
If the mother is receiving care from an OB-GYN, the pregnancy can be monitored 42 weeks while waiting for the onset of labor. However, monitoring does not decrease the chance of stillbirth.
Induction at 41 weeks does not guarantee a baby with no further problems. Many studies that examine risk of death for post term babies look at the risk of stillbirth and death in the immediate neonatal period. This skews the results to make it seems as though an induction will solve all problems related to post term pregnancy. It is possible that even with an induction at 41 weeks, some post term babies could later pass away from birth related complications or SIDS. It is possible there is some underlying condition related to a delayed onset of labor that is responsible for the increased risk of death for these post-term babies.
There is still a lot we don’t understand about post term babies and the onset of labor. Since there are still so many questions, it is very difficult to make a “one-size-fits-all” recommendation for post term pregnancies. The decision of whether to induce or not should probably be made on an individual basis by the mother and her health care provider.


Rosenstein, M. G., Cheng, Y. W., Snowden, J. M., Nicholson, J. M., & Caughey, A. B. (2012). Risk of Stillbirth and Infant Death Stratified by Gestational Age. Obstetrics and Gynecology, 120(1), 76–82.



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