I discovered this when trying to rapidly replace an ageing (and vanishing) population.

Drop a female villager onto a male (or vice versa)
Once you get a "going indoors" message, check the villager details of the female. If she's going to conceive the status indicator will already read "Nursing". Now pick her up and drop her. The baby will now appear after a few seconds.

Babies without all that messy, time-consuming "going indoors" stuff

It's probably a mild bug, but a really useful foil to "The Vanshings".

Enjoy