The original תקנה of שובבים ת"ת (fasting & Tikunim the
8 weeks starting from Parshas שמות) was intended only
for leap years.
The Levush O. Ch. 685 writes," one of the reasons we fast on
שובבים ת"ת so that pregnant women shouldn’t miscarry.
During leap years, it is more common for women to miscarry.
We fast 8 times for the last 8 months of pregnancy.
(first month [40 days] is not considered a miscarriage) .
It is appropriate to start on Parshas שמות, where it
says ובני ישראל פרו וישרצו "