Chances of Going Into Labor at 35, 36, 37, 38+ Weeks

This article is an excerpt from the Shortform summary of "Expecting Better" by Emily Oster. Shortform has the world's best summaries of books you should be reading.

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As you cross the 36th week of pregnancy, you’re past the preterm period (hooray!) but then you start worrying the baby will never come out.

The baby is normally due at 40 weeks. What are the chances of going into labor slightly early, at 36 weeks, 37 weeks, or 38 weeks? Find out here.

18% of babies are born week 38, 30% on week 39, and 17% on week 40. In sum, 70% of babies are born before their due date. So this is quite normal.

Here’s the chance of pregnancy for every week you reach:

Week of pregnancyChance of birth this week if still pregnant this week
42100% (induced)

Starting at week 37, your doctor will measure your cervix for a few predictors of labor:

  • Cervical thinness (effacement)
    • Women who were >60% effaced at 37 weeks were 98% likely to go into labor before due date. <40% effaced meant almost none went into labor before due date.
  • Cervical dilation
  • Baby drops down in pelvis a few days/weeks prior to labor
  • Bishop score – combines the above into a score from 0 to 13. > 6 is fairly advanced

Bishop score and cervical effacement are predictive of vaginal deliveries.

At 42 weeks, you’re almost certain to have labor induced, likely with Pitocin (synthetic oxytocin), which stimulates contractions. This is to reduce the risk of stillbirth with babies who are born too late. Oster argues that Pitocin may increase pain in labor and increase risk of C-section, which is why she was willing to wait until the very last moment to induce. 

Chances of Going Into Labor at 35, 36, 37, 38+ Weeks

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  • Why much parenting advice you hear is confusing or nonsense
  • The most reliable way to conceive successfully
  • How much alcohol research shows you can drink safely while pregnant (it's more than zero)
  • The best foods to eat, and what foods you really should avoid

Allen Cheng

Allen Cheng is the founder of Shortform. He has a passion for non-fiction books (having read 200+ and counting) and is on a mission to make the world's best ideas more accessible to everyone. He reads broadly, covering a wide range of subjects including finance, management, health, and society. Allen graduated from Harvard University summa cum laude and attended medical training at the MD/PhD program at Harvard and MIT. Before Shortform, he co-founded PrepScholar, an online education company.

2 thoughts on “Chances of Going Into Labor at 35, 36, 37, 38+ Weeks

  • September 14, 2021 at 12:31 am

    But do your figures of when babies are born include inductions or elective c-sections? If so, these are not very important figures in understanding when labor may start.

  • May 23, 2022 at 1:54 pm

    My EDD was 22 May2022 but still no signs


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