The womb lining (Endometrium) changes thickness during menstrual cycle.
During menstrual cycle the thickness of the womb lining (Endometrium) increases several folds (2-16mm). This was discovered recently. However 1400 years ago the Quran said that Allah knows every decrease and increase to the womb.
[Quran 13.8] Allah knows what every female bears, and every decrease and increase to the wombs. With Him, everything is by measure.
The thickness of the womb lining (Endometrium) depends on the stage of the cycle but 1400 years ago it was impossible to detect.
How could an illiterate man who lived 1400 years ago have known that there are changes to the womb?