The timeline

Pregnancy care runs on a clock. Here's where each step sits, and what's already closed when care starts late.

Move the slider to the week a pregnancy first reaches prenatal care. Watch which windows are still open.

The windows are not opinions. They are the weeks in which the trials that produced the evidence enrolled and treated. Outside the window, the evidence doesn't apply. Where a step has been tested outside its window, the result is on its card.

Week of first prenatal visit week 10

Nationally, 76.1% of mothers began care in the first trimester (by about week 13) in 2023 validated. The rest started later, or not at all. On a phone, the chart scrolls sideways.

open at that week partly open — late, but not too late closed — the window has passed event, not a scheduled window

The windows, one by one

Each carries the grade its evidence earns on the same bar: at least one adequately powered randomized trial, or an individual-participant meta-analysis of trials, showing a significant improvement in a preterm-birth band or a neonatal outcome, in a defined population. The grades are explained on the Sources page.

The clock the first visit has to beat

Read the chart from the left. Aspirin, where it is indicated, is best started before 16 weeks. The mid-pregnancy risk screen is drawn between 18 weeks and 20 weeks 6 days. A short cervix is found at the anatomy scan, 18 to 24 weeks, and treated from that day. By the time a pregnancy that entered care at 22 weeks has had its first visit, the first two windows are gone and the third is closing.

Our argumentA pregnancy that enters care in the first trimester can be offered everything on this chart. One that enters at 24 weeks can be offered almost nothing that prevents a preterm birth, only things that soften one.

That's why when care starts may matter more than any single step on the chart: it decides which steps are still possible. From January 2027, every prenatal visit is coded on its own, with its own date. The calculator puts a number on it for your state.

What is not on the chart, on purpose

17-OHPC (Makena). de-adopted Weekly injections for women with a prior preterm birth were the standard of care for a decade. The confirmatory trial did not replicate the benefit; the pooled estimate crosses null (EPPPIC 2021); the FDA withdrew it in April 2023. It is listed on the Sources page because a bar that only shows winners has no teeth.
Doulas, group prenatal care. Both have real value for engagement and experience of care. Neither has shown a reduction in preterm birth in randomized trials, so neither is credited with one here.
Anything whose window is not known. If the record does not say when it has to happen, it is not on the timeline.