When the time arrives, the mother will go into labor. It will last about three hours and the mother can do anything she wants during this time. She can eat, have conversations, go to the bathroom, even sleep. She'll have a +40 Uncomfortable moodlet during this time, however, which is too large to really overcome by normal means. If you like, you can avoid long labor by simply clicking the option to deliver at hospital from the Bassinet. The father in my game would sometimes have pre-parental panic and run through the house. He had the Hates Children trait, so that was a possible cause, but other fathers may experience this as well. Nobody else in the house (teens and children) had any reaction.
When it's Go Time, the mother will go to the bassinet for the actual birth, or you'll be taken to the hospital for surgery with the Get to Work Expansion (if you chose the hospital route). This is when we get to see her faces and listen to her groan. It doesn't last long and before you know it, you've got a brand new baby. The little darling will be dressed in a one-piece jumper: green for boys and yellow for girls. No traits are chosen at the time of birth, so it doesn't matter if the mother has a smooth pregnancy. The first trait is chosen at Toddler and a childhood Aspiration is chosen at the Child stage