Late-Term Pregnancies – Induction of Labour is Safer than Waiting until Post-Term

A recent Swedish study compared the safety of inducing labour at 41 weeks to waiting until 42 weeks before inducing. The study was stopped early due to the significantly higher rate of stillbirths in the group that was not induced at 41 weeks. Visit our late-term pregnancies program doc.

A normal pregnancy is, on average, 40 weeks. Pregnancies can be categorized as full term, late-term or post-term. “Full-term” pregnancies are between 39 weeks and 40 weeks, 6 days. “Late-term” pregnancies are between 41 weeks and 41 weeks, 6 days. “Post-term” pregnancies begin at 42 weeks. Adverse outcomes for mother and baby gradually increase after 40 weeks and are substantially increased post-term.[1] Once a pregnancy is post-term, the risks to the baby include fetal macrosomia (being significantly larger than the average), meconium aspiration, cerebral palsy, neonatal encephalopathy (brain damage), need for respiratory assistance, and death at the end of pregnancy or shortly after birth.[2]

Recommendations for the management of pregnancies in the late-term period are varied. For late-term low-risk pregnancies in healthy women, one of two approaches can be considered. Women can be induced as soon as they hit the 41-week mark, as is recommended by the World Health Organization,[3] or they can be monitored for signs of fetal or maternal complications and await spontaneous onset of labour, if no complications arise. The Swedish study compared the safety of these two options.

The study compared the two groups by evaluating Apgar scores and umbilical cord blood gases.  The authors also compared the incidence of hypoxic-ischemic encephalopathy, intracranial hemorrhage, convulsions, meconium aspiration syndrome, mechanical ventilation within 72 hours and brachial plexus injury. These outcomes were similar in women who were induced at 41 weeks and those who were monitored while awaiting spontaneous labour between 41 and 42 weeks.[4] The number of major maternal complications was also similar between the two groups.[5]

What was different between the two groups with the rate of perinatal death. There were no stillbirths or deaths in the first 30 days after birth for women induced at 41 weeks. In contrast, there were five stillbirths and one baby, large for its gestational age, died from lack of oxygen in the group that was monitored while awaiting spontaneous labour in the late-term period.[6] In addition, in the group that was induced at 41 weeks, fewer babies had to go to the neonatal intensive care unit, fewer had jaundice requiring treatment, and fewer infants were significantly larger than average.[7]

Based on the results of this study, the authors recommend that the induction of labour should be offered to women at 41 weeks or earlier.[8]  These results differ slightly from guidelines that have been in place in British Columbia for over a decade. [9]

The BC Reproductive Care Program (BCRCP) recommends that until 41 weeks, induction should be recommended only if there are signs of fetal compromise (babies that are too small or have a non-reassuring fetal heart tracing). At 41 weeks, all women with a healthy, uncomplicated pregnancy should be informed of the risks and benefits of induction versus expectant management (monitoring while awaiting spontaneous labour) and offered an induction. If the mother declines induction, the following monitoring should occur:  daily fetal movement counts, fetal heart rate testing twice a week, and ultrasound to assess amniotic fluid twice a week. If the fetal heart rate tracing or amniotic fluid is abnormal, the woman should be induced immediately. If spontaneous labour is not achieved by 42 weeks, labour should be induced.   

If earlier delivery is safer, should women be induced when they reach the full-term (39 weeks) or should induction wait until 41 or 42 weeks?  Clinical care practices and guidelines evolve over time as evidence accumulates. Some physicians are beginning to advocate for induction of labour at 39 weeks, rather than waiting for 41 or 42 weeks.  Look for our blog on these developments in the weeks ahead.

[1] Wennerholm et al, “Induction of labour at 41 weeks versus expectant management and induction of labour at 42 weeks” (BMI 2019; 367:l6131), p. 1

[2] British Columbia Reproductive Care Program, “Obstetric Guideline 7: Postterm Pregnancy”, p. 5

[3] Wennerholm et al, supra,  p. 1

[4] Wennerholm et al, supra, p. 3 and 10

[5] Wennerholm et al, supra,  p. 1

[6] Wennerholm et al, supra,  p. 8 and 10

[7] Wennerholm et al, supra,  p. 10

[8] Wennerholm et al, supra,  p. 11[9] British Columbia Reproductive Care Program, “Obstetric Guideline 7: PostTerm Pregnancy”

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Lindsay McGivern

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