Is Lise Bolskonski pregnant for 18 months?!

Hello,

I've recently started reading War & Peace. Not finished yet, so I don't want to read through the topics in case it spoils,y reading. So apologies if it's been mentioned already :)
A Google search brought me here regarding Natasha's age, which I believe is linked to the problem I noticed.

Lise is pregnant when Prince Andrew leaves for war. She is described as "waddling" so I'm assuming she is already quite far along. Anyway, when the war is over, Tolstoy says Nicholas Rostov has been away for 18 months, so I'm assuming it was the same for Prince Andrew more or less, given that he was also missing.

So he comes home and his wife is giving birth... 18 months after he left?! Is it a mistake in the book or something I misunderstood?

Thanks :)

Another thing I wondered: Countess Rostov is portrayed as an old woman, but she says she was married at sixteen. She has a daughter in her mid twenties, but that still makes her in her forties.

Another thing, Tolstoy say that force is mass times velocity, but I am pretty sure that is momentum. Force is mass times acceleration.