Sholom Rav,

I am not sure what your source is that Leah became pregnant from Yaakov with Bi'ah Rishonah. (On the contrary, the Moshav Zekenim of the Ba'alei ha'Tosfos on Parshas Vayechi says explicitly that the conception of Reuven (the firstborn) was Bi'ah Sheniyah and specifically not Bi'ah Rishonah. -Y.S.)

Moreover, the verse implies that when it became evident that Leah was hated, Hashem opened her womb. So even if the conception did occur from Bi'ah Rishonah, in this case it was through Divine intervention, as it was Hashem who opened her womb.

Be'Virchas Kol Tuv.

Eliezer Chrysler.