If you reach the population limit of the vault it should be IMPOSSIBLE for the girls to get pregnant. Maybe say that the Overseer puts contraceptives in the food.
BUT the couples should still be able to have six once per day to increase their happiness... but in that case the girl never conceives once the vault is full. Currently if the vault is full and a dweller gets pregnant or is already pregnant they STAY that way until you kill off another dweller. Until that point the pregnant female is basically useless (and isn't that sixist... how many women out there still work until they give birth).
Also maybe there should be a random number tested to see if normal, pre pop limit six gets the girl pregnant as well. If they don't get pregnant perhaps they should only get 50% of the happiness.