Chromosomes. I have an XX, my husband has an XY.
Biology/anatomy. uterus, vagina, breasts, etc. vs. penis, testicles
Hormones. Different levels of estrogen, androgen, testosterone, progesterone, etc.
I don't totally agree with him being able to change his sex legally. I guess it just doesn't make sense to me. I can't go and tattoo/dye my whole body a darker color and then legally call myself "African American" or "Middle Eastern". If you're 40, but only feel like you're 25, you can't legally change you're age. You're 40. That's it. I just don't get why sex should be any different. I know there are those whose sex is a bit more ambiguous, but for the general public, if you have an xx, a uterus, and carry/birth babies, you're a woman. You can dress however you want, have people call you whatever you want, take hormones, cut/grow your hair, get plastic surgery, etc, but at the core you're still the same person. I'm glad he's happy for who he is, I'm glad he's having healthy babies, I just don't agree with him legally wanting to be "male" and "father".
ETA: I hope this didn't sound offensive, it's just something I've thought about and I just don't understand it. I'm open to receiving any information that may change my mind.