The theory of "broken first child" is based on the "four-class" system.
After the Yuan Dynasty unified the whole country, the people of the whole country were divided into four grades: Mongolians, Semu people, northerners and southerners. Since then, the Mongols have exercised extremely cruel rule over the Han nationality and other ethnic minorities. Among them, we know that the eldest son is the most important to the Han people. However, due to the brutal rule of the Mongols, it is said that southerners can't enter the bridal chamber with their husbands when they get married, but have to wait on their Mongolian masters for a few days and nights first.
Therefore, after his wife comes back, the Han people will suspect that she is pregnant and fear that the child born is not their own, so the saying of "throwing the first child" has been handed down.
The idea of "breaking the first child" is actually a fallacy.
This statement is ridiculous. Although it is an indisputable fact that the Mongols ruled the Han people very savagely, the Mongols would not do such absurd things. And there is no such record in any history books in China. Such a statement has appeared in some novels in Europe, and it is likely to be invented by Europeans.