Why do I always want to have a baby?

I'm 19, and for the last year or so I have been REALLY wanting to have a baby. I see a lot of girls on Facebook who I went to high school with and they are all popping out kids. There have been at least 7 this year so far. They post all these pictures, and it makes me jealous. Also, the new ******* iphone commercial is a woman telling her man via video that she's pregnant. GRRRR!!!! I want to have a baby sooooooo bad!!! But my fianc� and I want to do things right. We want to wait 2 years to graduate college before getting married, and we've decided that we want to start baby making around 28 yrs old. But every few months I just want to have a baby. I even have dreams, and the love that I feel for this baby is immense, then when I wake up...it sometimes feels like I DO have the baby...but I don't. It makes me sad. Is this some sort of maternal instinct or bullshit??? Do all girls around this age just wanna be moms??? Is this why all these girls are baby making? ...maybe they're just dropout sluts...