In the Chinese culture of the setting of Pearl S. Buck's The Good Earth, there is more value placed upon boy children as they can help with the land, carry on the family name, and be valuable to the family. But, the birth of a girl child was considered bad fortune since they were only raised to belong to another family.
When Wang Lung enters the room where he sleeps with his wife, he observes that her "time" has come. O-lan speaks feebly to him,
"It is over once more. It is only a slave this time--not worth mentioning."
The announcement that they have a girl bodes badly for Wang Lung as he superstitiously feels that a girl has caused the trouble between him and his uncle.
Then, in Chapter 9, the reader learns that the little girl does not develop normally. For, she does not sit up as she should, lying uncompaining hour after hour wrapped in an old quilt. Also, she in now quiet, sucking feebly at whatever is in her mouth, and never using her voice:
Her little hollowed face peered out at them all, little sunken blue lips like a toothless old woman's lips, and hollow black eyes peeering.
Her father Wang Lung calls her his "poor little fool" and he feels such pity for her that he thrusts her inside his coat to warm her little cold body. When he does this, a faint smile crosses the little girl's face and it "broke his heart." And as the years pass, Wang Lung has a tender affection for his mentally handicapped daughter.