In my experience multiple doses do help them to have a baby. After 2-4 doses the couple is almost guaranteed to get a baby when they try for one.

However, I have noticed no increase in multiple births, even with 10+ doses for each peep (mother and father), so I do not believe it works to increase the chance of multiples at all.

I give baby boost to everybody. Hey, I what else am I going to spend $ on, now that I have all the upgrades?