Constructivism in this context says that apart from very specific things, like actually carrying the baby, differences in male and female populations doing things like caring for babies, or working less time, etc, are down to a society that expects that, either in terms of attitudes or how they structure institutions.
So, a simple example is that people who take this approach will sometimes argue that instead of maternity leave a couple should have a parental leave that they must share equally. They will tend to emphasise childcare rather than a parent at home so both can work.
Basically, if the conditions in society were really equal they feel there would be no difference in choices the male population and the female population would make.
To be frank though, the underlying idea that men and women are the same in interests is unproven and ideological - there is some evidence it's not the case and that women as a population do have more inclination toward care work.
And it tends to ignore that the really physical elements of reproduction - pregnancy, post-pregnancy, breastfeeding, hormonal effects of these things, etc, may themselves lead to different views on career, values, childcare, and so forth, between the male and female population.
As a result, it's left with a narrow set of possibilities discussing solutions, and those solutions tend to be ones that focus on outcomes for men and women being broadly the same in terms of things like workforce employment.