I've alway wondered the following: Since, in rich countries, nearly everyone who wants kids can have kids and if we assume a genetic factor in being drawn towar

I've alway wondered the following: Since, in rich countries, nearly everyone who wants kids can have kids and if we assume a genetic factor in being drawn towards kids, then there would be evolutionary pressure towards wanting kids. If this is possible, could we already detect this? If one would assume such an hypothesis and model population growth, when could changes be detected?