I have always tried to get my Sims pregnant just before having a few days off, because they've been getting paid, even if they weren't scheduled to work, from the very beginning.
Now, I usually have my Sims fairly high in their career, which would mean they'd have vacation days accrued, before they ever got pregnant. If your Sims actually weren't getting paid, this may be the reason.
I've heard it said (presumably here) that Natural Scientists at the top of their career (Ecological Guru) can earn $70,000 a week from well-timed pregnancies. I'm under the impression that,
if the sim is pregnant on a day she'd normally work, then she will be paid maternity pay for 3 days following plus the "stay at home & look after new baby" pay for the next 3 days. Thus if your Ecological Guru goes to work pregnant on Wednesday (her only work day), she'll then get the next 6 days paid - even though she wouldn't normally work on these days.
I suspect that people who are finding their sims aren't getting the full 3 days of paid maternity are also people who are getting their sims pregnant on a non-work day
. While it makes sense to do this so the sims get a day to rest (especially with the pregnancy mystery choking death bug that is supposedly now fixed), it must be stopping them from getting a "free" day's pay. That's my guess, anyway.
I'm not sure what happens with pregnant men - they don't normally get maternity pay, do they? But they do get the "stay at home for childcare" vacation.