According to the paper "Intended and Unintended Births in the United States: 1982–2010" where they interviewed 12 000 women, for women aged 15-24 either married

According to the paper "Intended and Unintended Births in the United States: 1982–2010" where they interviewed 12 000 women, for women aged 15-24 either married or living together with their partner, a little more than half of all pregnancies were intended. I think it's safe to assume that most of those were first pregnancies.