I have never had that situation, at the beginning of the game, i have babies as needed untill i have gotten level 3 farming, then i take 2 females and put them on parenting, and just leave them be untill they get to 50, then when that happens, i put two more on parenting, etc etc, when i have all the collectables reached, and im able to have the pop max, i put on one more female. i always make sure there are three mothers, after some time it usually works out so that i have one mother in early 20s, and one in her 30s and one in her 40s, as one turns 50, a newbie is brought in. this keeps my villages at a stable healthy 70-90 range, given twins and triplets maxed out. It works for me, and i always have a good balance of female to male ratios.