Sims are pregnant for 2 days, after you are forced to change their clothes due to their growing belly, so about 2 1/2 days total.
WooHoo is not the same as Try for Baby, so if the option to try for baby doesn't come up, there are 2 issues that prevent pregnancy: 1. in another relationsip/married 2. no room in household, which if the sim isn't actually living in the town in a visitable house, will say there's no room in the household.
Keep trying for baby until you hear the joyful, happy bells song, as the sound effect signifies pregnancy in both human sims and pets. Eat 3 apples for boy babies, watermelons for girls, and watch plenty of cartoons combined with the Fertility Treatment Lifetime Reward for multiples
Whatever...