When I looked up causes of death in Nigeria, malaria blew away anything maternal related[]. Not that I would want to die of either.
Another big one was HIV/AIDS. I guess it depends on cultural factors whether early marriage might reduce the number of partners that could introduce HIV/aids. If non-married people are less monogamous it's conceivable the increased risk of HIV/AIDS could overpower the risks of whatever additional childbirth is associated with marriage.
Also note pollution was one of the bigger risks present in Nigeria. So as people get educated to go slave away in a dirty factory (or a city full of them where educated people work) it might actually be worse for their health than staying at home and marrying into some pastoral herding tribe or something.
And more roads means more pollution. It is questionable if the answer is “make everyone dependent on cars”, although doing so obviously improves some outcomes.