What I've read recently, the better levels of Vit D of the mother, the baby will have stronger bones too.,
I was very low in 2006, 16, and took 4000 iu's of D3 for months and many months later finally got up to 80, and then after a hip replacement in 2010, levels dropped 40 pts which I knew could happen and it did.
So now I take usually 4000 IU's on day, 8000 IU's some days alternate this amount usually.
Latest research is that Vit K2 is needed for best Vit D absorption. J (76 yrs)