J., does your husband have a history of mood swings, instability, prolonged irritability and unreasonable thoughts?
Does he often punish you for insignificant issues? Or just huge, life changing events, like the birth of his very own baby?
You need to call a local woman's shelter and look into finding legal counsel for yourself and your future.
Your husband must not know anyone's legal rights very well, and you must not either, as you seem prone to believe him on this bizarre statement.
Does he often throw around odd and totally unenforceable ultimatums like this? At his family?
I wish I could connect the dots here for you, but you have to do that. Your MIL takes your 14 to NYC and gets her belly button pierced and your husband, now soon to be ex, is all OK with that. And now less than year later you're talking divorce. He and his family sound like people who don't like anyone telling them what to do, but they can toss out strange rules? It all sounds like too much drama and not much respect going around.
I'm so sorry that you are faced birthing this new baby alone. I hope you'll reach out to supportive family and friends and find someone you can count on and who doesn't threaten your well being like this.
Can you think of why you have tolerated someone not respecting you and it leading to this scenario? Did you have any ideas your husband was capable of being this mean to you during a very critical time in your life and your children's lives? This is all very sad and I truly pray that you find strength and peace through out the next phase of your life.
Please, please seek legal guidance and know your rights. Don't let him dictate to you what your rights are. He is manipulating the truth and trying to make you believe his nonsense. Don't do it.