I'm trying to make my game kind of realistic so I'm wondering if there are any fertility mods out there...(making some sims fertile and if possible shows you if your sim is fertile) Thanks

All adults female sims are fertile untill 5 days before aging up to elder. I think i saw a mod that increases the chance but at this time i can not remember.

ACR will allow you to change the % but it has a lot more to it.

I forget which EP it came with---but if your sim is a family sim---you can use his/her aspiration rewards to increase fertility....this will increase chances of getting pregnant and also of having twins. Also some modded objects like Christianlov's easy bed have the ability to change settings---you can choose to switch from Maxis default percentage chance of pregnancy whoohoo to I believe 100% chance of pregnancy.

I thought it was three days.

It takes 3 day from start to finish and the 3rd day can not be the last day of adult hood so to that the female must have 5 days left before elder-hood to become pregnant.

If you're okay with it, with InTeen the female has a biological clock where she can start a fertility treatment. And the expansion pack the Aspiration Perks came with is Freetime.

http://cyjon.net/node/320

It's probably me but I don't understand this part.

Day 3 of the pregnancy cannot be the day that the female is scheduled to age up to Elder at 6 pm. I assume that's because theoretically she could age up at 6 and give birth at 7, but then she'd be a pregnant Elder for an hour, and the game can't handle that.

If you want something temporary you may try some fertility/infertility potions:
http://www.modthesims.info/d/188471

Okay, I get it now. Thanks.
And all that time I was watching the 'until three days till elder', but I could've stopped 'worrying' at 5
(in the cases when I didn't want new babies, but they could decide on that themselves)