Importance  The 1980 and 1986 Swedish so-called speed premium policies aimed at protecting parents’ income-based parental leave benefits for birth intervals shorter than 24 and 30 months, respectively, but indirectly encouraged shorter birth spacing and childbearing at older ages, both risk factors for several perinatal health outcomes. Whether those policy changes are associated with perinatal health remains unknown.

Objective  To evaluate the association between the 1980 and 1986 speed premium policies and perinatal health outcomes.

Design, Setting, and Participants  This cross-sectional study investigated data from 1 762 784 singleton births in the Swedish Medical Birth Register from January 1, 1974, through December 31, 1991. Data were analyzed from October 11, 2022, to December 12, 2023.

Interventions  Speed premium policy introduction (January 1, 1980) and extension (January 1, 1986).

Main Outcomes and Measures  Total population register data were used in an interrupted time series analysis with segmented logistic regression to calculate the odds of preterm birth, low birth weight, small for gestational age (SGA) at preterm, and stillbirth measured before and after the speed premium policy reforms. Subgroup analyses by maternal origin were conducted to evaluate changes by different policy responses.

Results  Among 1 762 784 births analyzed, 4.8% were preterm (of which 12.0% were SGA), 3.2% had low birth weight, and 0.3% were stillbirths. The 1980 speed premium policy was associated with a 0.3% monthly increase in the odds of preterm birth compared with the period before the reform (odds ratio [OR], 1.0029 [95% CI, 1.002-1.004]), equivalent to a 26.4% increase from January 1, 1980, to December 31, 1985. After the 1986 relaxation of the policy, preterm birth odds decreased 0.5% per month (OR, 0.9951 [95% CI, 0.994-0.996]), equivalent to an 11.1% decrease across the next 6 years. Low birth weight displayed a similar pattern for both reform periods, that is, increased 0.2% (OR, 1.0021; 95% CI, 1.001-1.003) per month in 1980 through 1985 compared with baseline, and decreased 0.3% (OR, 0.9975; 95% CI, 0.996-0.998) per month in the following period, but was attenuated when considering low birth weight at term. Odds of SGA at preterm were decreased after 1980 (OR, 0.9965; 95% CI, 0.994-0.999) but not in 1986 (OR, 1.0009; 95% CI, 0.998-1.003), whereas stillbirths did not change following either reform (1980: OR, 1.0020 [95% CI, 0.999-1.005]; 1986: OR, 1.0002 [95% CI, 0.997-1.003]). Subgroup analyses suggested that perinatal health changes were restricted to births to Swedish- and Nordic-born mothers, the primary groups to adjust their fertility behaviors to the reforms.

Conclusions and Relevance  Despite its economic advantages for couples, especially for mothers, the introduction of the speed premium policy was associated with adverse perinatal health consequences, particularly for preterm births. Family policies should be carefully designed with a “Health in All Policies” lens to avoid possible unintended repercussions for fertility behaviors and, in turn, perinatal health.