Interpregnancy Interval and Risk of Adverse Pregnancy Outcomes: A Register-Based Study of 328,577 Pregnancies in Denmark 1994-2010.

Objectives To investigate the associations of interpregnancy interval (IPI) with miscarriage, stillbirth, preterm delivery and small for gestational age delivery. Methods The study population comprised all women who had a live birth and at least one subsequent pregnancy in Denmark during the period from 1994 to 2010 (N = 328,577). Linear regression was used to estimate risk differences for miscarriage, stillbirth, preterm delivery and small for gestational age delivery according to IPI. Results The results were heterogeneous: the risk of miscarriage increased monotonically with the length of the IPI. Compared to women with IPIs of 18-23 months, women with IPIs of 0-5 months experienced 18.7 (13.1-24.2) fewer miscarriages per 1000 pregnancies, while women with IPIs of ≥ 60 months experienced 28.7 (23.4-34.0) more miscarriages per 1000 pregnancies. We found that women with IPIs of ≥ 60 months had 1.7 (0.4-3.0) more stillbirths per 1000 births compared to women with IPIs of 18-23 months. U-shaped associations were seen for preterm delivery and small for gestational age delivery with women with IPIs of 18-23 months experiencing the lowest risks of these outcomes. Conclusions for Practice The heterogeneity in associations between IPI and adverse pregnancy outcomes suggests that different mechanisms of action may be at play at various times in the antepartum period. While the finding for miscarriage suggests that fecundity is an important determinant for IPI, the findings for preterm delivery and small for gestational age delivery suggest the coexistence of the maternal depletion syndrome mechanism and the physiological regression mechanism and the finding for stillbirth speaks against a strict maternal depletion syndrome explanation.