Birth Control causes more babies.

The Family in America

"widespread use of contraception and the availability of abortion changes relationships between men and women in significant ways. These changes do not just affect the individuals who choose to use the technology. Contraceptive technology, imposed on the states by the Court and subsidized by Title X and Medicaid, changes the terms on which everyone participates in the dating and mating game, even if they decline to use the technology.

When sexual activity entails a high probability of pregnancy, society develops a set of cultural norms for inducing men to commit to women they impregnate. These norms, largely if informally enforced by women, include a strong social expectation that women will refuse sex outside of marriage. Women develop a set of socially acceptable ways to refuse sex, even with men they genuinely like. The norms also include the expectation that the man will marry a woman he impregnates. All these norms are designed to discourage casual encounters and encourage fathers to commit to mothers.

But when low-cost, highly effective contraception is introduced, distributed, and paid for by the federal government, these cultural norms no longer serve the same purpose and are likely to fade. A woman cannot so easily resist a man’s sexual advances on the grounds that she might get pregnant: the probability of pregnancy is lower. The man can make birth control and sex the price of being in a relationship with him. In many ways, the man has the upper hand; he can more easily find an alternative lover if she refuses. Even a woman who would be inclined to abstain from premarital relations will face increased pressure to be sexually permissive. The pressure comes not only from men but also indirectly from competition with other women."

Yep. That's what happened.