THE DANGER OF OVERMATCHING IN STUDIES OF THE PERINATAL-MORTALITY AND BIRTH-WEIGHT OF INFANTS BORN AFTER ASSISTED CONCEPTION

Objective: To examine the impact of gestation length and plurality on short-term outcome of in vitro fertilization (IVF)-children. Study Des ign: A register study using the Finnish Medical Birth Register for 199 1-1993 (N = 194 383 newborns, of which 1335 were IVF-newborns). Result s: For IVF-newborns, a high proportion of multiple births (27%, odds r atio (OR) 19.67 compared with non-IVF births), low birth weight infant s (30%, OR 7.94), and perinatal deaths (2.9%, OR 4.17) was found. The mothers' background did not explain the increased risks. After adjusti ng for gestation length and/or plurality, the odds ratios decreased si gnificantly. Conclusion: Because of the high risk of plurality and pre mature births, matching the control group by gestation length and/or t he number of multiples births may yield misleading results on the tota l health impact of IVF, and therefore it should be avoided.