NEW YORK (Reuters Health) – Treating high-risk pregnant women with a progesterone hormone during the second and third trimester to prevent preterm birth seems to be safe for the fetus, according to a report in Obstetrics & Gynaecology. High levels of progesterone are produced by the placenta, which helps maintain the pregnancy. A drop in the levels of this hormone is thought to be one of the steps that stimulate the onset of labour. Treatment with 17 alpha-hydroxyprogesterone caproate has been shown to reduce the rate of repeat pre-term births at less than 37, 35, and 32 weeks of pregnancy, the authors explain, but it remains unclear whether such treatment is safe for the foetus.