Child births, ovulation and pregnancy.

In 2016, the number of abortions for this age group was about the same as the number of births.

Ovulation dictates pregnancy. So young girls can get pregnant in the weeks before their first period as they could release an egg before then. Ovulation is a bit irregular generally in those first few years.

Same holds true for adult women too. Women who for whatever reason aren't menstruating regularly might ovulate unexpectedly and get pregnant. Same is possible for trans men.

For all three groups it can be especially challenging as they may not realize they are pregnant for some time, as an absence of a period wouldn't be unusual for them.