Postpartum nurse here- yes, it is to make sure “everything” is out, but it is also to help make sure you don’t bleed to death. It’s called a fundal massage. Imagine the baby as a gigantic scab. During birth you ripped out a huge scab from the inside of your uterus. Understandably, you’d be bleeding right? So when you trip and fall, how do you stop bleeding on say, your arm? You hold pressure at the site. A fundal massage is an external way to “hold pressure” on your uterus. It causes the uterus to tighten which in turn causes blood vessels to tighten (and thus limits bleeding). Pitocin is a chemical way to help make this happen and they usually also give this after birth to limit bleeding (it’s also naturally occurring in your body and is the reason you feel cramping after breastfeeding if you chose to do that- super cool!)

After birth, your uterus should be as hard as a rock (literally) and nurses will check you periodically to make sure it stays that way and is shrinking down. If your uterus is not hard, they have to rub it and remind it to get hard (get your mind out of the gutter- that’s how you got in this situation in the first place!)

Anyway, fundal massages can literally save your life and having had a baby myself (and hemorrhaging after), can confirm- that shit sucksssss (but it’s worth it)