Objective 
To describe the number of admissions of pregnant people to U.S. jails and the outcomes of pregnancies that end in custody.

Methods
We prospectively collected pregnancy data from six U.S. jails, including the five largest jails, on a monthly basis for 12 months. Jails reported de-identified, aggregate numbers of pregnant people admitted, births, preterm births, cesarean deliveries, miscarriages, induced abortions, ectopic pregnancies, and maternal and newborn deaths.

Results
There were 1,622 admissions of pregnant people in 12 months in the selected jails. The highest 1-day count of pregnant people at a single jail was 65. The majority of these admissions involved the release of a pregnant person. Of the 224 pregnancies that ended in jail, 144 (64%) were live births, 41 (18%) were miscarriages, 33 (15%) were induced abortions, and four were ectopic (1.8%). One third of the births were cesarean deliveries and 8% were preterm. There were two stillbirths, one newborn death, and no maternal deaths.

Conclusion
About 3% of admissions of females to U.S. jails are of pregnant people; extrapolating study results to national female jail admission rates suggests nearly 55,000 pregnancy admissions in 1 year. It is feasible to track pregnancy statistics about this overlooked group.