@HelenP Formerly in my immortal dynasty household of 6 sims, I moved in an elder to prevent multiple births, then had the female sim get pregnant.
That elder died before the pregnant sim's due date, however, and she gave birth to twins.
For this reason, I think that giving a fertility massage very late in the pregnancy makes more sense.
But even with fertility massages and both parents having the fertility trait, I don't consistently get multiple births.