Vit B6 would be of more use than B12 for lengthening luteal phase. But it's not a good idea to take a B vitamin in isolation as they all need to balance each other. A B complex would help. But I don't think there's much point in just doing that until you've established your cycle. OPKs and temping, using a tracking program, is really the best way.
ooppsy i meant B6,
i had a look through my TTC journal and yeh it was B6 =)
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