baby pressure
Maybe the reason the church tells you to pop them out early is because at that point you're still young and naive, and haven't had time to fear it.
This is definitely the spirit in which I had my son. Way too fast, way too young. And we had even been married for years by the time I had him! I've had people at grad school ask me, "Um, I don't mean to be offensive, but was he planned?"
"Yes, he was," I say. "We were just young and crazy."
I remember feeling the push to bear children while I was at BYU living in married housing. The people without kids were referred to strangely enough as "single people." They were outside the social circles, never quite accepted. The ones with kids had nothing to say to them. I felt like I was getting really old and had waited way too long to get pregnant when I was 21 and had been married for about a year.
When I became pregnant while living there, I didn't tell anyone. I was rebelling against that system of motherhood-is-everything. When I started showing, though, at about 16 weeks, damn my belly, I was suddenly in the in-crowd. Oh, goodie me. The women really had nothing else to talk about but babies and scrapbooking; those were the only issues on which they talked and connected. Even then, I hated that culture, and felt relieved that we moved out of there before the baby was born. Not a single one of my new "friends" tried to follow up when the baby was born. I was in a different ward, you see, all the way down the street.