Chances of live birth after exposure to vitamin D-fortified margarine in women with fertility problems: results from a Danish population-based cohort study

OBJECTIVE: To study the association between extra vitamin D from a mandatory margarine fortification program and chance of live birth among infertile women.

DESIGN: Nationwide cohort study.

SETTING: Not applicable.

PATIENT(S): The study population consisted of 16,212 women diagnosed with infertility from June 1, 1980, to August 31, 1991.

INTERVENTIONS(S): We took advantage of the mandatory vitamin D fortification program of margarine in Denmark that was abruptly stopped on May 31, 1985. The termination of the vitamin D fortification served as a cutoff point to separate the study population into various exposure groups.

MAIN OUTCOME MEASURE(S): Odds ratios and 95% confidence intervals for the association between vitamin D exposure status and chance of a live birth within 12, 15, and 18 months after first infertility diagnosis.

RESULT(S): Women who were diagnosed with infertility during the vitamin D-exposed period had an increased chance of a live birth compared with women diagnosed with infertility during the nonexposed period. For women diagnosed with infertility during the wash-out period, the chance of a live birth was also increased, but somewhat lower. Similar estimates were obtained with longer follow-up, in women with anovulatory infertility, and little seasonal variation was observed when calendar period of conception was applied.

CONCLUSION(S): Our findings suggest that infertile women exposed to extra vitamin D from a margarine fortification program had an increased chance of live birth compared with women not exposed to extra vitamin D from fortification.