Linked to stress.
Researchers have long known that during times of stress, the brain releases several hormones — including one called corticotropin-releasing hormone (CRH). In past research, women who deliver prematurely or have low-birth-weight babies were often found to have high levels of CRH in their bloodstream, and other studies show a greater risk of miscarriage in women reporting stress. CRH is a hormone the brain secretes in reaction to physical or emotional stress, and it is also produced in the placenta and the uterus of a pregnant woman to trigger uterine contractions during delivery.
But this new research suggests that CRH and other stress hormones may also be released elsewhere in the body, where it specifically targets localized mast cells — those best known for causing allergic reactions. Mast cells are abundant in the uterus. During stress, the local release of CRH causes these mast cells to secrete substances that can cause miscarriages.
WebMD, How Stress Causes Miscarriage, 2003
Severe stress can lead to miscarriage. However this was portrayed in the Quran 1400 years before it was discovered. In the final Hour every pregnant woman will miscarry.
[Quran 22:2] When you will see it [final Hour] every nursing mother will discard her infant, and every pregnant woman will miscarry, and you will see the people drunk, even though they are not drunk. but the punishment of Allah is severe.
The final Hour will cause every pregnant woman to miscarry. Today we know why, severe stress causes miscarriage.