Baby self-weaning while I am pregnant?

Hi.  I have a 12 month old and am 19 weeks pregnant.  I have noticed a considerable dip in suppy recently and my daughter is not nursing nearly as much as she was even a few weeks ago (suppy dip started first, but I am sure her new pattern is not helping).  I think she will probably stop nursing all together pretty soon.  Is there a way to encourage a not as interested baby to continue nursing?  Will she really pick it back up after the new one is born or is this the end?