A pregnant woman’s estimated due date is 40 weeks after the date of conception. Even without a premature delivery, however, not all women will deliver at 40 weeks. Forty weeks has the highest percentage of deliveries, on average, in comparison to any other length of pregnancy, but there is a wide range for how long a pregnancy lasts. Term pregnancy ranges from 37 weeks 0 days to 41 weeks 6 days. Once a woman reaches 42 weeks, she is classified as post term. Within term pregnancies, there are now a few subclassifications. Early term is the period from 37 weeks 0 days to 38 weeks 6 days. Full term is 39 weeks 0 days to 40 weeks 6 days. From 41 weeks 0 days to 41 weeks 6 days is considered late term.
In a previous blog article, I discussed the current British Columbia guideline for induction of labour in late term versus post term pregnancies, and the medical evidence supporting this guideline. The current standard of care in British Columbia is to continue pregnancy (absent signs of fetal or maternal compromise) until 41 weeks and then offer the woman an induction. The standard of care requires that women be informed of the risks and benefits of induction versus expectant management (monitoring while awaiting spontaneous labour) and allowing the woman to make the best choice for her and her child. For post term pregnancy, the standard of care is to induce.
Guidelines, however, are constructed based on the current, widely accepted, medical knowledge of various conditions and situations. This medical knowledge is constantly growing, changing, being confirmed or getting disproven. As knowledge is accumulated, standards of practice evolve. In the context of pregnancy, an example of this is the shift in recommendation from offering induction of labour to women only when they reached post term (42 weeks) to offering induction of labour at 41 weeks. Will the evolution of pregnancy guidelines continue to progress towards even earlier delivery? Some physicians are now advocating for induction of labour at 39 weeks. This subset of physicians believe that the safest option is to deliver the baby as soon as the mother reaches full term.
One such physician, Dr. Errol Norwitz MD, PhD, MBA, recently presented on this topic at the 36th Annual Obstetrics, Gynecology, Perinatal Medicine, Neonatology and the Law Conference.1 In considering whether to continue a pregnancy beyond 39 weeks, a physician and his or her pregnant patient must consider the risks of continuing the pregnancy and balance that against the potential benefits (to both mother and baby) of continuing the pregnancy. Let’s start with the induction of labour itself. One benefit to continuing pregnancy would be to potentially avoid the induction by having the woman go into labour spontaneously. There are some risks of induction of labour, but Dr. Norwitz argued, with studies to reinforce his position, that the risks of routine induction of labour are lower than previously thought. One of the risks is a risk of failed induction leading to caesarean section. Newer studies cited by Dr. Norwitz, however, have shown that there is no increased caesarean section rates in elective induction of labour at 39 weeks. The ARRIVE trial by Dr. William Grobman (and others) actually showed a lower rate of caesarean section with elective induction of labour at 39 weeks compared to expectant management until at least 41 weeks.
Dr. Norwitz argues that there are no benefits to continuing pregnancy beyond 39 weeks sufficient to overcome the risks associated with doing so. Benefits to a baby in continuing pregnancy before 39 weeks are a result of the decreasing risk of complications associated with prematurity. This decrease in risks, however, stops at 39 weeks. Dr. Norwitz cited a study by Dr. Brian Mercer showing the frequency of a baby developing problems at each gestational age. The risk of sepsis (infection), respiratory distress syndrome, intraventricular hemorrhage (a brain bleed), necrotizing enterocolitis (a disease affecting the intestines), retinopathy of prematurity (a potentially blinding disease), bronchopulmonary dysplasia (chronic lung disease) and need for hospital stay are all at their lowest at 39 weeks. Dr. Norwitz also looked at a study by Dr. Joann Petrini (and others) showing that the risk of cerebral palsy, developmental delay and seizures are also at their lowest point at 39 weeks. Now, some of these risks remain at that same low level throughout 40-42 weeks. Others rise, in terms of risk, at 40 weeks. None, however, ever get lower than they are at 39 weeks.
So, the risks of known, common complications do not get lower than they are at 39 weeks. What about the risk of unexplained death or stillbirth? Dr. Norwitz cited studies by Dr. Christina S. Cotzias (and others) and by Dr. Ruth Fretts showing that the number of stillbirths is far greater than the number of Sudden Infant Death Syndrome (SIDS). Dr. Norwitz cited numerous studies showing that the risk of stillbirth increases after 39 weeks. Stillbirth is the death of a baby during pregnancy or during delivery. If a woman’s baby is assessed to be alive and healthy at 39 weeks, and is delivered at that time, the risk of stillbirth drops to the risk that the baby will die during the delivery. If the pregnancy continues, there is a continuing risk of stillbirth. It may be a small risk, but any risk is higher than the risk of stillbirth at 39 weeks when the baby was assessed to be alive. In other words, if a baby has been delivered at 39 weeks, it cannot die in utero at 40 weeks. That risk of stillbirth disappears. A 1985 study by Drs. George Feldman and Jennie Freiman includes the following quote: “[b]etween 1 in 50 and 1 in 500 fetuses reach maturity in utero and then suffer a catastrophic event leading to permanent neurological injury or death.”2 If a baby has survived and is healthy at 39 weeks, full maturity, why do we accept any risk of stillbirth by continuing the pregnancy beyond that point?
As mentioned above, the current standard of care according to the British Columbia guidelines is to offer induction of labour at 41 weeks, not before. Medicine and knowledge evolve with time, however, and this is one area where some physicians are advocating for further change.
- Norwitz, Errol R. 2019. “Optimal Timing Of Delivery: Should It Be 39 Weeks And Out?”. Presentation, 36th Annual Obstetrics, Gynecology, Perinatal Medicine, Neonatology and the Law Conference, 2019.
- Feldman GB, Freiman JA. N Engl J Med 1985; 312:1264-7