אשה מעוברת בשנת העיבור

The Levush Orach Chaim 685 writes, one of the reasons we fast  on Shovevim Tat , so that  pregnant women shouldn’t miscarry. 

During a leap year, it is more common for women to have miscarriages.

We fast 8 times for the last 8 months of pregnancy. (first month [40 days]
is not a miscarriage) . It is very appropriate to start on Parshas Shmos, where it says ובני ישראל פרו וישרצו --