For a lot of woman in "somehow develoed countries" the labour market is today more attractive than the "marriage market" as women rights etc. give them much more freedom than it was in older days (e.g. in some EU countries the woman had to ask their spouse if they allow them to work up into the 70s).

If you are for women rights etc., then you have to accept that this includes much lower birth rates (as having childrend is not the only way to survive).

Birth rates are only up in countries without any social development of women rights.