The contribution of twins conceived by in vitro fertilization to preterm birth rate: observations from a quarter of century.

Objective: Little information exists related to the contribution of assisted reproductive technology (ART) twins to the preterm and very preterm birth rate. We sought to examine this contribution over a period of more than two decades in a tertiary perinatal center. Methods: We identified all preterm births from 1993 to 2017, born at <37 or <32 weeks' gestation, by mode of conception [in vitro fertilization (IVF) vs. non-IVF pregnancies]. We generated trend lines of the annual change of the dependent variable (% preterm birth). Results: We evaluated 74,299 births, including 3934 (5.3%) preterm births at <37 and 826 (1.1%) at <32 weeks' gestation. In this period, 1019 (1.4%) twin pairs were born including 475 (46.6%) and 80 (7.8%) at <37 and <32 weeks, respectively. There were 213 (5.4%) IVF pregnancies among the preterm births at <37 weeks, including 88 (41.3%) twins. Fifteen (1.8%) births of all IVF gestations were at <32 weeks, and all were twins. Whereas the annual rate of spontaneous twins did not change, a significant increase over time exists for IVF twins (P<0.05, R-2 =0.6). We demonstrated an increase in IVF twin births at <37 weeks but not for spontaneously conceived twins. Whereas the twin birth rate at <32 weeks did not change over time, all preterm births at <32 weeks following IVF were twins. Conclusion: The risk of twins after ART increasingly contributes to preterm births at <37 weeks and ART twins are at significant risk for preterm births at <32 weeks.