Sims 2: Pregnancy: When two Sims love each other very much…
Female Pregnancy
Sim fertility has been a big topic of mine recently, since I’m doing a generational challenge (follow a line of Sims through 26 generations, one for each letter of the alphabet). The basic facts of Sim life are simple: when two romantically involved Sims are together on a property with a double bed, click on the bed and tell your Sim to “Relax.” If you are in control of both Sims, tell them both to Relax. If you are only in control of one Sims, have it relax on the bed, and most likely the other will follow.
Once they are in place on the bed, click on the other Sim and choose the menu option “Try For Baby.” If “Try For Baby” is successful, you will hear a little lullaby sound effect play after your Sims have finished their, err, exertions.
A Sim has to be older than teen to get the bed options. If you have the University expansion pack, your Sims can engage in hanky panky while they are at college. (Once you have upgraded them to a double bed, of course.) Sims have to have fallen in love before they will engage in either “Woo Hoo” or “Try For Baby.”

I have read that the “Woo Hoo” option does NOT include a risk for pregnancy. Can anyone confirm or deny this? At any rate, “Try For Baby” is not 100% guaranteed. Interestingly enough, I have found that the odds for “Try For Baby” seem to vary by couple. Out of the three couples I have set to generating heirs, three had 100% success with “Try For Baby” each time. (I’m having each heir give birth to three babies: an heir, a spare heir, and a spare spare heir.)
The third couple, Connor and Kana, have a little more trouble getting pregnant. I have had them “Try For Baby” seven times, with three resulting pregnancies. This means that “Try For Baby” is only 42% effective for this particular couple. I’m not sure if that is a glitch in the game, a quirk of the random generator, or if some Sims are just less fertile than others.
Naturally, a double bed is not the only place that your Sims can “Try For Baby.” The option will also appear in the photo booth, the car (if you buy one for your family), the hot tub, and changing rooms at clothing stores. (Naughty!)
Same sex Sim couples will not get the “Try For Baby” option. If you wish to obtain a baby for your same sex Sims, have them phone the adoption agency and request an adoption. Note that your Sims will need to be financially solvent in order to be approved for an adoption! (I have had Sims turned down for adoption in the past.)
Male Pregnancy
Yes, male Sims can get pregnant! But the odds are very low. If a male Sim is abducted by aliens, there is a small chance that they will return home having been impregnated by the alien Pollination Technician. After three Sim days, the male Sim will give birth to an alien baby.