About 4 percent of women incarcerated in state prisons across the U.S. were pregnant when they were jailed, according to a new study released Thursday that researchers hope will help lawmakers and prisons better consider the health of women behind bars. The number of imprisoned women has risen dramatically over the past decades, growing even as the overall prison rates decline. But there had been a lack of data on women's health and no system for tracking how frequently incarcerated women were pregnant, or what happened to the pregnancies. The Bureau of Justice Statistics, for example, collects data on deaths in custody but not on births. Dr. Carolyn Sufrin of Johns Hopkins University School of Medicine attempted to fill the void by collecting data from 22 state prison systems and 26 federal prisons during a yearlong period in 2016 and 2017. She released her results in the American Journal of Public Health . 'The fact that nobody had collected this data before signals just how much this population is neglected,' Sufrin said. There were 753 live births among the 56,262 women included in the study, with about 10,000 in federal prisons but the majority in state prison systems. There were 46 miscarriages, 11 abortions, four stillbirths and three newborn deaths, according to the study. No women died during childbirth. Among women who were already in state prisons, five new pregnancies were diagnosed during a six-month period — three women became pregnant during work release and the other two were not reported. Researchers found there were 1,396 women who reported being pregnant while incarcerated — 1,224 from state prisons and 172 in federal prisons. The researchers found differences by state. Texas and Ohio, large states with large prison populations, had some months when there were more than 50 pregnant women jailed. Other states had months with no pregnant women. Overall, about 6 percent of pregnancies resulted in miscarriage, but in some states that was as high as 20 percent, according to the study. March of Dimes, a nonprofit organization that works to improve the health of mothers and babies nationwide, estimates that between 15 and 25 percent of recognized pregnancies overall end in miscarriage. Brenda Baker, a professor and researcher at Emory University School of Nursing who teaches prenatal care to pregnant women who are incarcerated in Georgia, said the research was much needed. 'We are so starved for data. The fact that someone can get something like this and share it excites us,' she said. 'Those of us who do research in this area will use it far and wide.' She said pregnant women have been a virtually unknown population in the criminal justice system. 'But women are the fastest growing sector of the prison population — women of childbearing age. If you can't measure it, you can't fix it,' Baker said. Most incarcerated women have to give up their babies within days of having them, especially if they are serving long sentences. In rare cases, like at the Bedford Hills Correctional Facility in New York, some women are allowed to keep their newborns in a separate nursery inside the prison run by a nonprofit. The study included about 57 percent of all women in prison — 53 percent from state prisons and 86 percent in federal prisons. There are about 112,000 women behind bars nationwide. The data was collected in states with large female prison populations, such as Texas, Ohio, Oklahoma, Arizona and Georgia, and in states with smaller populations like Vermont and Wisconsin. But some large states declined to participate, including California, Florida and New York. Designated reporters, which included prison employees as well as health care personnel, reported monthly. They did not collect data on the women's health, socio-economic status or prior pregnancy history — factors that could influence a pregnancy outcome.