There's no way they are in the womb (they wouldn't be able to get inside the uterus, and the immune system would attack them quite quickly if they did).
They are almost certainly transfered via close skin contact (breastfeeding, etc).
"There's no way they are in the womb (they wouldn't be able to get inside the uterus, and the immune system would attack them quite quickly if they did)." That is simply untrue [1,2,3,4, 5]. It has been recently discovered that many fetuses acquire their gut bacteria before parturition, it is logical to assume that the same can happened with these mites.
They are almost certainly transfered via close skin contact (breastfeeding, etc).