What is a Post-Term Pregnancy?
Post-term pregnancy is defined as pregnancy lasting 42 completed weeks or more. Post-term pregnancy, in most cases, probably represents a variant of normal, and is associated with a good outcome, regardless of the form of care given (1). Why are we concerned about a post term pregnancy? There is a slight increase in the risk of the baby dying during labor, birth or shortly after when pregnancy continues past 42 weeks. In a very small number of babies, for reasons that are not fully understood, the placenta does not deliver adequate nutrition and oxygen to the baby especially after 42 weeks of pregnancy. This is called Post Maturity Syndrome. These babies do not have the reserves to tolerate the process of labour. Research found that induction of labor at 41 weeks, compared to waiting for labor to start on its own, lowered the death rate for babies from 2.5/1000 to .3/1000 (2). Other concerns associated with prolonged pregnancy are an increased risk of meconium (the first bowel movement) in the amniotic fluid, cesarean delivery, newborn seizures and large baby (with increased risk of birth injury).
Prevention of Post-Term Pregnancy
Research has shown that rimming the cervix when pregnancy is at or beyond term reduces the need for medical induction. Rimming the cervix is the separation of the membranes from the cervix and lower part of the uterus with the finger during a vaginal exam. There is no evidence of an increased risk of infection. It is associated with the side effects of increased bleeding and irregular contractions.
Management of Post Term Pregnancy
Research shows that routinely inducing labour after 41 weeks of pregnancy reduces the occurrence of the death of one normally formed baby for every 500 inductions that are done. There is no decrease in meconium aspiration syndrome or seizures in the newborn. The rate of cesarean section is unchanged. There is no evidence of benefit when inductions are done at less than 41 completed weeks of pregnancy.
When monitoring post term pregnancy, the evidence does not show any advantage to the use of complex fetal monitoring such as a biophysical profile over the simpler non-stress test and measurement of amniotic fluid pool depth. However, the medical standard in Calgary is to do an ultrasound for a biophysical profile. This includes an assessment of the volume of amniotic fluid. At 41 weeks of pregnancy you will be offered a biophysical profile to assess the baby’s well being. Given the available evidence, it is reasonable at 41 weeks of pregnancy for the mother to decide whether she wants to be induced or to wait for labor to start on its own. A home birth is not recommended where the pregnancy continues past 42 completed weeks.
For further information we recommend:
1. Enkin, M, et. al. (2000) A guide to effective care in pregnancy and childbirth, 3rd ed. University Press: Oxford.
2. Barrett, J. & Pitman, T. (1999). Pregnancy and birth: the best evidence. Key Porter: Toronto.
Accepted by the Division of Midwifery, Calgary Health Region, 21/02/11