The fertility difference you're seeing is just the standard liberal-conservative fertility divide.It's a reasonable assumption, and there is of course an effect from political orientation that is distinct--in the sense that we understand these things as being distinct, not necessarily distinct in terms of causation for which our understanding is incomplete at best--from fertility. Liberals with kids are considerably more pro-choice than conservatives with kids are, and liberals without kids are considerably more pro-choice that conservatives without kids are.
But the effect is still quite pronounced even among women who share a political orientation. The following graph shows the percentages of white women who believe abortion should be legal in any circumstance by the number of children they have and by political orientation. For contemporary relevance are responses are from 2000 onward (n = 2,408):
GSS variables used: ABANY, RACECEN1(1), YEAR(2000-2014), POLVIEWS(1-3,5-7), CHILDS, SEX(2)