That you get pregnant as soon as you DTD with our contraception (admittedly it didn't take long for us, but I honestly used to think once and bam!)
That miscarriage exists and is common. I don't think I ever really heard about it growing up. I knew about it when I got pregnant the first time, but definitely didn't expect it as I didn't realise it was so common.
That childbirth isn't all screaming like in the movies.
That breaking waters isn't necessarily the first sign if labour.