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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A premature birth occurs before 36 weeks and occurs in 12% of pregnancies. Early-term is between 37-38 weeks, and full-term is 39 and beyond.
Over the past decades, survival for preterm babies has increased dramatically due to improvements like assisted ventilation (lungs develop later in pregnancy). It’s a marvel that even babies born at 22 weeks still have a shot at survival.
What are the specific chances of survival for each week of pregnancy? How likely is a baby to survive at 24 weeks, vs 26 weeks, and 28 weeks?
Despite their chance of survival, premature children are more likely to get sick, have lower IQs, and may have vision/hearing problems. 75% of 5-year-olds born before 30 weeks have a disability (vs 27% of full-term births). Their IQs were 5-14 points lower.
Here’s a table of % of births occurring at each week of gestation:
| Weeks of Gestation | % of Births | Probability of Death in First Year |
| 22 | 0.05% | 77.1% |
| 23 | 0.06% | 62.6% |
| 24 | 0.09% | 39.3% |
| 25 | 0.10% | 26.0% |
| 26 | 0.11% | 18.1% |
| 27 | 0.12% | 13.6% |
| 28 | 0.17% | 7.5% |
| 29 | 0.20% | 5.5% |
| 30 | 0.28% | 4.0% |
| 31 | 0.36% | 3.2% |
| 32 | 0.51% | 2.1% |
| 33 | 0.78% | 1.5% |
| 34 | 1.39% | 1.1% |
| 35 | 2.33% | 0.8% |
| 36 | 4.37% | 0.6% |
| 37+ | 89.09% | 0.2% |
Chances of a preterm baby are thus sizable, but still in the small minority.
The overall story is, premature babies can be born at a <10% chance, and they have a good rate of survival.
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Here's what you'll find in our full Expecting Better summary :
- 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
