The contribution of twins conceived by assisted reproduction technology to the very preterm birth rate: a...

To assess the contribution of ART twins to the very preterm birth rate in Slovenia.

In this retrospective population-based study we stratified by plurality and gestational age (<32 weeks vs >32 week) all deliveries >24 weeks in women who conceived by ART. We counted the frequencies of all ART liveborn twins among all liveborn infants and among all liveborn twins and frequencies in the subgroup of very preterm births.

During the period 1987-2010, there were 13,293 twins (2.73%, range 1.93-3.62%, a nearly 2-fold increase from 1987 to 2010), 6939 infants born after ART (1.42%, range 0.03-2.62%, an 87-fold increase from 1987 to 2010), including 2317 (33.4%, range 14.28-44.83%, a 3-fold increase from 1987 to 2010) twins (0.47% of all deliveries). A total of 425 twin infants (0.99%, range: 1.07-1.2%, insignificant increase) were born at <32 weeks' gestation, including 261 after ART (61.4%, range 20-100%, 5-fold increase). Twins after ART born at <32 weeks comprised 0.05% of all births (range 0.004-0.11%) and increased 27-fold from 1987 to 2010.

The incidence of twins after ART born at <32 weeks increased 27-fold from 1987 to 2010 and has not reduced from its peak incidence over the last decade.