In The Sims 4, babies can be produced using Try For Baby or using the computer to adopt.
Adoption costs $1,000 and the player gets to choose between a baby, toddler, and a child. You also get to pick the one you wish to adopt. The computer will show pictures and names of the available babies, toddlers, and children. Click on the one you want. My Sim chose a baby named Savannah. She immediately walked out of the house and disappeared at the street. About an hour later, you'll get a notice that your Sim is bringing home a new family member. You'll also be given the option to change the name of the baby you pick. Your Sim returns with some confetti and the baby is magically in the bassinet. Adopting a toddler or child works the same way.
CARING FOR THE BABY
The baby cannot be the active Sim and the player cannot see his/her needs. You have to judge the situation from visual and audio clues. but they have a hidden hunger, fun and hygiene need that must be met. The baby will sleep automatically when well cared for. You can develop the relationship by using the limited socials that are available, while also satisfying the baby's needs for entertainment and socialization. A green cloud means a dirty diaper and crying means the baby needs to be fed and/or you should played with the baby. My Sims were fairly attentive to the baby without being told, but this might not always be the case. All interactions are found by clicking on the bassinet and the baby never leaves that immediate area.
When feeding, the mother has the option to bottle feed or breast feed. Everyone else can only bottle feed. Males pregnant with an alien baby can breast feed, however. The other interactions besides changing the dirty diaper are: Bounce, Coo At, Make Silly Face, Rock, Talk, Shoosh, Show Off To, and Age Up. Children will be able to Coo At and Talk to the baby. To use the Show Off To interaction, make sure the bassinet is not up against a wall. The person looking at the baby will stand at the head of the bassinet.
Leaving the House and Neglecting the Baby
Sims can all leave the house when there's a baby, no problem. The baby automatically gets sent to Day Care while the parents and others are away at work, or just out exploring to collect items. If a baby is neglected for a period of time, the baby will be taken away to safety. The baby will simply disappear in a green flash of light. This didn't seem to negatively impact the mother's ability to have more babies later.
Aging Up to Toddler
A baby can be aged up to Toddler at any time after the birth by clicking on the bassinet. However, if you wish to wait until the normal aging time, you may wait it out by caring for them until this happens automatically. The only real benefit to letting the baby age on its own is to increase the relationship between the baby and other Sims in the house, while also giving you time to prepare to care for a toddler properly by giving them their own bedroom. All of the interactions your Sims can make with the baby will increase the relationship bar. When the time comes, the baby will age straight to a toddler who can roam the house and needs to learn skills. The duration of the baby life stage will vary depending on which lifespan you're playing. On the short lifespan, it's about 1 1/2 days to age up to toddler. On normal lifespan, it's about 3 days. My baby seemed to be bugged on the long lifespan because he was getting close to two weeks and still hadn't aged up. We'll update this later with more definitive information on babies on long lifespan.