There are no pre-created, playable Sim babies in the series to date; all pre-created Sims younger than toddlers are unborn babies when first played.
Ruby Ann Hatfield's bio in The Sims implies that she is pregnant with her fourth child. However, since pregnancy was not a game mechanic in first generation Sims games, this is not actually possible, and it is up to the player whether or not she has another child through interactions with her husband.
In order for Brandi Broke to have had enough time to grow up to be an adult and a parent herself in The Sims 2, Betty Newbie would presumably have had to be pregnant with her by the start of The Sims. However, as stated above pregnancy is not a mechanic in the original game, and Brandi's time-line does not quite fit in with the original even if she was born soon after the start of The Sims. (As she would be, at most, twenty-five years old according to the game lore, but has a son who is at least thirteen, something that is barely achievable even with hacks to allow teen pregnancy in The Sims 2.)
The three unborn babies in The Sims 2 base game all have unique attributes among unborn Sims in the series: unborn baby Broke is the only one whose gender is pre-determined (he is always male unless cheats are used); unborn baby Curious is the only alien-induced pregnancy (and Pascal is the only pregnant male); and unborn baby Monty is the only unborn Sim to be deceased (due to the mother's death during her pregnancy).
Another unique attribute attaches to Samantha Ottomas's unborn twins: they are the only pre-determined multiple birth for a human Sim in either The Sims 2 or The Sims 3.
Canonically, Unborn baby Monty of Monte Vista and Unborn baby Capp of Roaring Heights are Claudio Monty and Kent Capp, respectively. However, both may be born male or female in The Sims 3, and the player is free to name them whatever they choose.
Pascal Curious's pregnancy also has a canonical outcome: The Sims 2 (PSP)reveals that his baby was a boy named Tycho. As above, however, the baby may be born male or female in The Sims 2 main game, and the player is free to choose the baby's name.
All moved-in households in The Sims 2 base game have a series of story prompts to get the player started. If these are followed, Vidcund Curious will become pregnant through alien abduction during the initial play-through of the Curious household. Though these events were scripted by the creators and therefore can be considered canonical, no information is given in the PSP sequel as to the identity (or the fate) of Vidcund's baby.
Unlike in its Sims 2 equivalent, The Sims 3: Pets includes no pregnant dogs, cats, or horses. There are, however, a small number of playable puppies, kittens, and foals present in the game when it is first played.
Though pregnancy is a game mechanic in The Sims 4, there have as yet been no pre-created pregnant or unborn Sims.
Two of the first generation console games, The Sims (Console) and The Sims Bustin' Out (Console), require the player's character to have children in order to complete all the game objectives. The player needs to have two children in The Sims and one child in Bustin' Out before the game can be won. The pregnancy and baby mechanics function as they do in the original Sims base game on PC, with babies appearing instantaneously as interactive objects that age up to children after 3 in-game days. As usual in first generation Sims games, the babies can be either male or female, twins are not possible, and the player is free to name the babies.