Prenatally is an adjective that describes anything that happens before a baby is born. However, if you want to use a synonym for prenatally, then you could use terms such as fetal, embryonic, or in utero. The term fetal refers to anything that happens when a baby is still in the womb; meanwhile, embryonic refers to the stage when the baby is still in the earliest stages of development. On the other hand, in utero describes anything that happens inside the uterus, which is where the baby resides during pregnancy. By using these synonyms, you can make your writing more varied and interesting, while still conveying the same meaning.