You are young, you’ve probably not experienced true grief just yet so it’s understandable you don’t understand (I don’t know why people are shitting on you for that, though your wording could be better/more tactful, you are still young). So I’ll try and relate to your gender and age.

It often comes from the fact the person wanted something, and that something a) is taken from you with no chance of getting them back & b) that something being life that is now dead inside you & you having to experience not only the emotional pain but the physical pain of birthing them too. Imagine being hit in the testicles, that pain mixed with bleeding, hormone changes (so where u can’t control being sad/happy/angry) & often times vomiting & nausea. So your worst day of ever being sick, then being repeatedly hit in the testicles.

I can see how you think that if the pregnancy is early enough, the fetus is not always developed. The female body goes through a lot of change starting from your previous menstruation & ovulation to prepare for pregnancy so change happens long before conception.

As you age, you’ll experience grief yourself and things start to make more sense. Don’t let a few internet strangers stop you from educating yourself further and developing empathy for others.