The Moontide Quartet series by David Hair
The world of Mage’s blood is very rich, with clearly defined differences between the various cultures but it is not subtle. The overarching plot of the Moontide is obviously heavily influenced by the 11th and 12th century Crusades and the cultures can be very easily divided into Eastern and Western as it would be done in reality.
The Moontide is an event which happens every twelve years. Two continents, Yuros and Antiopia, are separated by water but connected by a bridge which is only passable during Moontide. The western continent uses this time to invade the eastern continent.
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There are three main characters: Elena Anborn, a mage protecting the Royal family in Javon, Alaron Mercer, Elena’s nephew, who is a student (and later failed) mage, and Ramita Ankesharan, a daughter’s merchant in Lakh; who is chosen by the mage who united the two continents with the bridge, Antonin Meiros to be his wife as her lineage is almost guaranteed to produce multiple births and he needs children. The chapters alternate between them and a few others, Gurvon Gyle and Kazim; for example.
And indeed, the first book ends with the Moontide about to happen.
This Reddit query specifically mentions her gaining magic through pregnancy.
A young woman living in a village is picked as a wife by an old wizard(I know, gross), when she gets pregnant by the wizard it gives her super magic somehow. She has a sister who later turns evil.