When babies actually arrive
Just over half of US babies are born at 39 or 40 weeks. Almost a third arrive at 37 or 38.
The forty-week due date sets an expectation that the data do not support. Here is the whole distribution, per live births, for 2024.
| When the baby was born | Share of 2024 live births |
|---|---|
| Early preterm, under 34 weeks | 2.72% |
| Late preterm, 34 to 36 weeks | 7.69% |
| Early term, 37 to 38 weeks | 30.26% |
| Full term, 39 to 40 weeks | 54.61% |
| Late term, 41 weeks | 4.47% |
| Post-term, 42 weeks or more | 0.25% |
Being born at 39 or 40 weeks is the single commonest outcome, but it is barely a majority. Nearly a third of babies come in the two weeks before that.
Preterm birth, and the part of it that has changed. The overall preterm rate went from 9.85% of live births in 2016 to 10.41% in 2024. That is roughly 378,000 preterm births in 2024, a figure worth treating as approximate because NCHS publishes the rate rather than the count.
The important detail is where the change sits. Early preterm birth, under 34 weeks, is essentially unchanged: 2.76% in 2016 and 2.72% in 2024. Late preterm birth, 34 to 36 weeks, rose from 7.09% to 7.69%. NCHS reports both: The 2024 early preterm rate is the lowest reported since 2020 (2.70%).
And: The late preterm birth rate has risen 13% since the most recent low in 2014 (6.82%).
ACOG's 2021 guidance says that approximately 1 in 10 newborns in the United States is born prematurely
, and describes the causes: approximately one half of preterm births follow spontaneous preterm labor, about a quarter follow preterm prelabor rupture of membranes (PPROM), and the remaining quarter of preterm births are intentional, medically indicated by maternal or fetal complications.
A quarter of preterm births, in other words, are decisions rather than events.
"If my baby comes at 36 weeks, what changes? What about 34?"
"Is my due date from an early ultrasound or from my last period?"