I'm not sure exactly what she's going for. She seemed to imply she wanted the alliance cemented by the conception of a child to unite the bloodlines, but with the low fertility of whampires that can't be relied on any time soon. In Blood Rites, the figure was that humans in a long-term relationship with a whampire usually wear out within a few years even with care taken not to drain them in one shot. So unless wizard and/or mantle markedly improves Harry's resilience to being fed on, Mab seems to be rolling the dice on whether this marriage idea is going to cost her Winter's starborn asset, maybe even before producing a child.
It took an infected Justine under 2 years and 2 months to get pregnant while trying.
We don't know how long she was trying really.
