Think about how the pregnancy will affect the characters' lives.
How will the mother feel when she finds out? How will the father feel? Does she even know who the father is? Will being pregnant keep her from doing her job? How will this affect the lives of others? You may want to make a chart of all the possible effects of her pregnancy.
Decide how the baby will be or was conceived.
Make it believable meaning don't make the girl wake up one day and suddenly she's pregnant any book written like this makes it sound like the author never passed health class there are a ton of ways now that a girl can get pregnant.
Decide how your character finds out she's pregnant.
Does she take a pregnancy test? Does her doctor tell her? Does she miss her period? Is she wounded in battle and the medics find out while using their equipment to check her wounds?
Decide what your pregnant character is like from the beginning of the story until the end.
Is she ready for a baby? How does pregnancy affect her, aside from just the baby bump? (Morning sickness? Hormones? Strange food cravings?) What does she look like in all three trimesters? Does she have any trouble giving birth?
Decide what the offspring will be like. This will be especially important if you intend to write sequels where the child is older.
Decide on the basis of the child - what do they look like? What's their assigned sex? Do they have any problems during the birth? Make sure that the child resembles the parents, but is not a 50/50 mixture of their traits. Genetics are complex; two people with brown hair can have a blonde child, and a family of tall people can produce a short child. Similarly, don't base the child's personality off of a mixture of the parents'. It's true that many traits can be passed from parent to child, but make sure you're giving the child plenty of qualities that are unique to the child. If you're dealing with characters who have powers, you could go in a lot of directions. Maybe the child inherits a parent's powers, only weaker. Or because they're half human, the powers don't show up at all. If your character is half-human or otherwise multiple species, how will they take on traits of either species?
Be cautious with certain scenarios
Be sensitive with disability and birth defects. It can seem like a good way to give the story a plot twist, especially in future sequels, but if you write it poorly, you may upset readers - whether it's because of the accuracy issues or because they're disabled themselves. Dramatic birth scenes may intrigue readers, but they can also be written terribly and disappoint readers. Don't try and make the scene too interesting, or it'll fall flat on its face. Very few birth stories involve ten complications at once.
Do your research.
It's important to put in as much research into this, as you would a school project. Research the basics of pregnancy, the effects of pregnancy on the body and mind, possible complications (if you want to write a complication), and how one needs to care for their pregnancy to keep the baby healthy. Research the effects of pregnancy after the parent has given birth, too. Nobody loses all their baby weight right when they give birth, and if you want your fanfiction to be accurate, you'll need to learn about what people would do to care for babies after birth in that time period, too. Be aware that some descriptions of pregnancy and childbirth can be quite disgusting. Prepare yourself!
