The danger of overmatching in studies of the perinatal mortality and birthweight 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 Design: A register study using the Finnish Medical Birth Register for 1991-1993 (N = 194 383 newborns, of which 1335 were IVF-newborns).

Results: For IVF-newborns, a high proportion of multiple births (27%, odds ratio (OR) 19.67 compared with non-IVF births), low birth weight infants (30%, OR 7.94), and perinatal deaths (2.9%, OR 4.17) was found. The mothers' background did not explain the increased risks. After adjusting for gestation length and/or plurality, the odds ratios decreased significantly.

Conclusion: Because of the high risk of plurality and premature births, matching the control group by gestation length and/or the number of multiples births may yield misleading results on the total health impact of IVF, and therefore it should be avoided.