Will a vitamin B12 deficiency cause infertility?

B 12 deficit. If a patient has been found deficient in vitamin B12 usually is due to poor absorption in the stomach because of lack of intrinsic factor. Then the replacement or treatment with B12 must be parenteral (intramuscular) or at least sublingual.
No. No, but b vitamins are important for healthy organ development in early pregnancy. It is important to take supplemental folic acid, and consider a b complex vitamin as well, to support normal organ development if you are reproductive aged woman.