If you are trying to have twins in Bitlife, it, for the most part, is left to RNG as there is no way to increase your chances of having twins via normal pregnancy in the game.
That being said, there is of course, a better and surefire way to have twins, and it is through the IVF and Artificial Insemination activity. You can find both activities in the fertility tab in Bitlife.
However, you will need to be a female character in order to use the artificial Insemination option. But on the other hand, the IVF option can be selected by both male and female characters, but you will be rejected if you are 50 years old.
Using both IVF and Artificial Insemination will also have a higher chance of getting you, twins. So because of this, we recommend using the option at least once every year until it happens. You can also reset your results if they are not favorable by closing the game app on your phone and then restarting it.
The trick to do this is at the moment when you get the prompt telling you the result. If the result it is bad, do not close the prompt or hit accept; instead, simply close the app and restart the game.