Parshas Toldos -

Parshas Hashavua

Midrash Rabah Parshas Toldos (65) quotes Koheles (11:5) כעצמים בבטן המלאה [you don't know] " the nature of the embryo in a

pregnant stomach." 

The עץ יוסף writes the reason we don't know the gender in a pregnant stomach is that otherwise, some רשעים would cause the woman to abort if the gender won't be to their liking.

As a result of female infanticide and sex-selective abortion combined, there are an estimated 30–40 million more men than women in China today. This female deficit is expected to generate a wide range of adverse social, political, and economic consequences