It is surprising how many people are deficient of vitamin D.
I live in Australia, so you'd think there's no way I'd need it. I get plenty of sun, but I'm actually a bit on the low side. My doctor tested me after my second miscarriage.
She said that she finds many people who are tested are actually low, and need supplementation. So I take it everyday - 1000iu or 25 mg. I have heard it helps prevent premature labour and other problems in pregnancy.
I don't think it can harm, and there is a chance it may help, so I think it is worth trying.