Post-Term Pregnancy

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What is “post-term”?

  • Recall the ACOG language surrounding pregnancy term status:

    • 24-27’6 weeks: extremely preterm

    • 28-33’6 weeks: preterm

    • 34-36’6 weeks: late preterm

    • 37-38’6 weeks: early term

    • 39-40’6 weeks: term

    • 41-41’6 weeks: late-term

    • 42’0 and beyond: post-term

  • Birth certificate data in the US indicate:

    • About 5% of births happen at 41 weeks and beyond

    • About 0.25% of births happen at 42 weeks and beyond

      • Largely a consequence of how we manage that today - since we tend to induce folks before they get there in many systems.

  • Risk factors for a patient being post-term include:

    • Nulliparous

    • Obesity

    • AMA

    • Personal history of post-term births in prior pregnancies

    • Male fetus (interesting)

    • Certain fetal conditions that likely interrupt signaling for parturition – i.e., anencephaly, placental sulfatase deficiency/X-linked ichthyosis 


What’s the problem with post-term pregnancies?

  • Think of the fetus “overstaying its welcome.” There are multiple consequences:

    • Macrosomia

      • 2.5-10% of post-term pregnancies have weights >4500g (vs only 1% at term)

    • “Dysmaturity”

      • Sign of chronic intrauterine malnutrition likely resulting from relative oligohydramnios, cord compression, meconium passage

      • Physical signs are what we think of for post-term pregnancies:

        • “Parchment-like” skin – less vernix, skin peeling, feels thin

        • Meconium staining of amniotic fluid and skin; increased risk of meconium aspiration

        • Long, thin bodies and long fingernails

        • Hypoglycemia

        • Polycythemia

    • Increased risk of perinatal morbidity/mortality

      • Fetuses born after 42 weeks have higher rates of NICU admission, seizures, meconium aspiration, and 5-minute Apgars under 4.

      • Oligohydramnios rates go up the longer pregnancy goes, predisposing to risks associated with oligohydramnios.

        • Remember, because of stillbirth risk, in isolation delivery for oligo is recommended anytime after 36 weeks!

      • Infants born at 41 weeks or greater also have about a one-third higher risk of perinatal death versus those born at term gestation.

        • This risk increases to:

          • 2x greater risk at 42 weeks; 

          • 4x greater risk at 43 weeks; and 

          • 5-7x at 44+ weeks.

      • Importantly, while these relative risks are quite high, it should also be stated the absolute risk is low:

        • The stillbirth risk at 40 weeks  is less than 1 per 1,000 ongoing pregnancies.

          • At 41-42 weeks, this increases to 1.2 - 1.3 / 1000.

          • At 42-43 weeks, 1.3 - 1.9 / 1000.

          • At 43-44 weeks, 1.5 - 6 / 1000. 

  • There are also maternal risks to ongoing pregnancy:

    • A large observational study has suggested increased rates of many major OB morbidities, including:

      • Higher order laceration

      • Postpartum hemorrhage

      • Infection

      • Cesarean delivery


How do I manage a post-term pregnancy?

  • One really important thing to always confirm (and a limitation to some of the data we’ve described) is accurate pregnancy dating.

    • Check out our previous episode!

    • Some studies have shown that the incidence of post-term pregnancy was reduced from 9.5% to 1.5% when switching from LMP to ultrasound-based dating – underscoring the importance of early ultrasound! 

      • This may ultimately help to decrease unnecessary intervention, whether that be for post-dates, incorrectly calling a fetus FGR, or delivering out of concern for macrosomia, amongst other issues.

  • Membrane sweeping gets mention in this practice bulletin!

    • Using a finger to “stir up” around the membranes at the internal os, releasing endogenous prostaglandins.

    • Cochrane review demonstrates it reduces incidence of pregnancies progressing past 41 weeks.

      • Use after a shared decision-making process with your patient.

      • Can be uncomfortable and cause some bleeding.

  • Fetal surveillance

    • ACOG recommends doing something for pregnancies progressing past 41 weeks, but stops short of making an outright specific recommendation.

      • This is because there are insufficient data comparing approaches for antenatal testing, and whether one reduces stillbirth risk better than another.

    • While ACOG does recommend antenatal testing, they also admit there are no RCTs demonstrating that fetal surveillance decreases perinatal morbidity or mortality. 

      • The benefit is presumed in capturing and reducing risk where we can (i.e., finding oligohydramnios).

    • Some options (might bring you back a bit to our antepartum testing episode — or use that as a refresher!).:

      • Biophysical profile (BPP) or modified BPP

        • A small RCT has not demonstrated any difference between the two in neonatal outcomes when used in pregnancies beyond 42 weeks.

        • Advantageous in that you are getting NST as well as a fluid assessment to check for oligo.

        • One of the better, less invasive tests for negative predictive value of stillbirth.

        • For fluid assessment, deepest vertical pocket has been demonstrated to reduce unnecessary intervention without increasing adverse outcomes for fetuses.

      • NST alone

        • More convenient to do and does not require ultrasound. 

        • Weekly or twice-weekly acceptable

          • Twice-weekly may be better based on some small studies, but data are insufficient – again, given the overall rare absolute outcomes of stillbirth or neonatal morbidity, these studies are difficult to adequately power. 

  • Labor considerations

    • ACOG doesn’t provide any specific recommendations regarding labor management in this population, aside from one group: TOLAC.

      • There does not appear to be increased uterine rupture rates when advancing beyond 41 weeks, but

        • Failure of TOLAC risk increases – from 22.2% before 40 weeks, to 35.4% after 41 weeks.

      • This can be a conundrum – especially because of data suggesting increased success with spontaneous labor (which isn’t guaranteed!) versus a slightly increased risk of rupture with induction (remember that episode on TOLAC?).

      • Working through a shared decision making process and being aware of all of the risks can help your patients make the right decision for them. 

  • How long can I let it go?

    • ACOG says that induction at 41’0 “can be considered” based on the limited evidence of increasing risk at/beyond this gestational age.

      • If advancing beyond 41’0, some sort of antenatal testing should be performed.

    • ACOG definitively recommends delivery at 42’0 and beyond, definitely by 42’6, due to the risks of morbidity/mortality.