Stratified risk of pregnancy loss for women with a viable singleton pregnancy in the first trimester

Objective Calculate the risk of miscarriage in women with a viable (defined as presence of fetal heart rate on ultrasound) first trimester singleton pregnancy and to create a model for stratified risk-assessment for pregnancy loss based on significant risk factors. Study Design Retrospective cohort study of unselected women with singleton pregnancies in a large obstetrical practice who presented for prenatal care prior to 14 weeks over a three-year period. All women underwent a formal first-trimester ultrasound, and we only included women with viable pregnancies with fetal heart activity seen on that ultrasound. Our primary outcome was pregnancy loss prior to 20 weeks. Statistical modeling was used to create a risk-assessment tool from adjusted likelihood ratios of pregnancy loss based on risk factors independently associated with this outcome. Results From January 2015-December 2017, 2,446 women met the inclusion criteria for the study and 132 (5.4%) had a pregnancy loss <20 weeks. On regression analysis, the independent risk factors for pregnancy loss were earlier gestational age (aOR 0.72, 95% CI 0.65-0.80) and increasing number of prior miscarriages (aOR 1.56, 95% CI 1.32-1.83). Using these risk factors, we calculated the stratified risk of pregnancy loss, which ranged from 0.8% in women at 13 weeks of gestation with no prior miscarriages to 33.7% in women at six weeks of gestation with three or more prior miscarriages. Conclusion In first trimester singleton pregnancies, the overall risk of pregnancy loss <20 weeks after confirmation of fetal heart activity is 5.4%, but can be stratified for each woman and ranges from 0.8% to 33.7% based on the gestational age and number of prior pregnancy losses.