Birth weight is lower for survivors of the vanishing twin syndrome: a case-control study.

Objective: To compare the outcome of singleton pregnancies originating from a twin gestation with singleton pregnancies originating from a single gestation.

Design: Case-control study.

Setting: Women's general hospital in Austria.

Patient(s): A total of 794 singleton deliveries after IVF/intracytoplasmic sperm injection, including 46 survivors of vanishing twin syndrome (study group) and 92 matched singletons (control group).

Intervention(s): None.

Main Outcome Measure(s): Data concerning duration of gestation, mode of delivery, birth weight, neonatal intensive care unit admission, duration of hospitalization, perinatal mortality.

Result(s): Of all singletons born, 5.8% originated from a twin gestation. Mean (+/- SD) birth weight was significantly higher in the control group (2876.3 +/- 600.5 g vs. 3249.6 +/- 624.5 g). The frequency of low birth weight (26.1% vs. 12.0%) and being small for gestational age (32.6% vs. 16.3%) was significantly lower in the control group. No differences were observed in terms of duration of gestation.

Conclusion(s): A higher risk for lower birth weight and being small for gestational age for survivors of the vanishing twin syndrome was found. Such pregnancies need to be carefully monitored, and parents must be informed about the associated risks when transferring more than one embryo.