• Nathan Riley, MD

Obgyno Wino Podcast Episode 70 - Management of Late-Term and Postterm Pregnancies

Updated: May 1

"The true mystic is both humble and compassionate, for she knows that she does not know.”

- Father Richard Rohr

Dark Side Red Blend from 7 Moons Wine

PB#146 - Published August 2014 (Reaffirmed 2019)

Five Pearls

  1. When pregnancy goes beyond 41 wga, there are increased risks for mom and baby, but absolute risk is overall still very low. these risks are still low in absolute.

  2. Pregnancy dating by LMP combined with early ultrasound is far more reliable than LMP alone.

  3. "Membrane sweeping" decreases the chance of a pregnancy going beyond 41 wga, but consent your patient first!

  4. If fluid checks out, particularly if BPP is otherwise reassuring, it's reasonable to continue pregnancy

  5. IOL at or beyond 41 wga does not improve fetal or neonatal outcomes apart from a possibly lower risk of meconium aspiration syndrome. NNT = 410 to prevent one perinatal death.

First, some definitions...

- Early term: 37 - 38w6d

- Full term: 39 - 40w6d

- Late-term: 41 - 41w6d

- Postterm: 42 wga or greater

Etiologies and risk factors

- no understanding to why some pregnancies go far longer than others

- from observational studies, risk factors include:

  • nulliparity

  • history of postterm pregnancy

  • male fetus

  • obesity

  • anencephaly

  • placental sulfatase deficiency

Risks to fetus/newborn in late-term or postterm pregnancy

- increased risk of neonatal convulsions

- meconium aspiration syndrome

- 5-minute APGAR score <5

- admission to NICU (odds ratio 2)

- 2x risk of macrosomia

- oligohydramnios (increased risk for umbilical cord compression, FHR abnormalities, umbilical artery blood pH of <7, meconium-stained fluid, and low APGAR scores)

- increased relative risk of stillbirth beyond 41 wga (odds ratio 1.5; 1.8 after 42 wga; 2.9 after 43 wga)

Risk of stillbirth per 1,000 births by gestational age (Source: Muglu et al, PLOS 2019)

Postmaturity syndrome

- 10-20% incidence

- decreased subcutaneous fat

- decreased vernix and lanugo

- meconium stained fluid, membranes and umbilical cord

Risks to mother in late-term or postterm pregnancy

- severe perineal laceration

- infection

- postpartum hemorrhage

- c-section, operative vaginal delivery, and shoulder dystocia (perhaps due to the increased risk for macrosomia)

Ensuring accurate dating is an important means of decreasing the incidence late-term and postterm pregnancy

- basing pregnancy dating on LMP alone is fine, but far more reliable when combined with early ultrasound

- "membrane sweeping" also decreases the chance of a pregnancy going beyond 41 wga; contraindicated in placenta previa or other contraindications to labor or vaginal birth (counsel your patient and get consent first!)

Note: Jury is still out on whether GBS colonization poses additional risks to the pregnancy.

Should I be recommending antepartum fetal surveillance for late-term or postterm pregnancy?

- Despite the increased relative risk of stillbirth after 41 wga, antepartum surveillance for isolated late-term pregnancy has not been shown through RCTs to improve outcomes

- reasonable to offer after 41 wga, however, after you've discussed risks/benefits with patient

- NST, CST, BPP, or mBPP are reasonable options if you proceed with surveillance

- type and optimal frequency haven't been clarified

- mBPP = BPP in efficacy (looking at umbilical cord blood pH)

- NST = BPP in efficacy (looking at perinatal death rate)

- a few small studies have suggested that twice weekly testing is better than weekly, but weekly is still likely sufficient

- monitoring amniotic fluid has greatest sensitivity for identifying late-term and postterm pregnancies at risk for adverse fetal outcomes (use of DVP < 2cm was associated with less unnecessary interventions without increases in adverse perinatal outcomes)

- oligohydramnios is associated with increased risk for fetal demise, meconium-stained fluid, abnormal FHR patterns, and growth restriction

- if fluid checks out, particularly if BPP is reassuring, reasonable to continue pregnancy

When should I recommend induction of labor?

- a famous multi-center RCT looked at the benefits of elective IOL at 41 wga versus expectant management

- no differences seen between the groups in terms of perinatal mortality or neonatal morbidity; higher c-section rate for the expectant management group

- the Cochrane group also looked at the optimal timing of induction for term and postterm pregnancies; 22 RCTs were included (9383 women), though they ascertained "moderate bias"

- they concluded that NNT (number needed to treat) through IOL to prevent one perinatal death was 410; they also conclued that at or beyond 41 wga, IOL carries lower risk of meconium aspiration syndrome and c-section; no difference in NICU admission rate

- conclusion: given the modest increase in risk of stillbirth, it's reasonable to consider IOL at 41 wga or later; it's recommended after 42wga and before 42w6d

- if your patient has a history of prior c-section, awaiting spontaneous labor carries decreased risk of uterine rupture (already a very low absolute risk); however, TOLAC success rates decrease with increasing gestational age

**For more information on this topic, please check out Rebecca Dekker's article on this topic at Evidence Based Birth.

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