How do miscarriages happen?

This is another very technical thing and while there are many reasons for which a miscarriage to happen, there a couple types of miscarriages that are more common than others. This article that I quoted from last night, really gives a good description of these different types of miscarriages:

“The most risky time is between six and eight weeks from the last menstrual period. Over half the babies who are miscarried during this period have a chromosomal abnormality. This occurs when the crossover of genes from the sperm and the egg takes place at the time of conception. Sometimes, some genetic information is lost and the pregnancy cannot continue. This is known as a ‘chance event’ and has no known medical cause. Exactly which information is lost determines when the miscarriage will happen. The point at which the information is needed, and is not there, is the point at which the baby stops developing and dies, and, usually, the miscarriage begins. This genetic information may be needed immediately, or not for some weeks, and the pregnancy will carry on as normal until that time.

The miscarriage may not happen immediately, leading to what is called a ‘missed’ miscarriage which may not be picked up until some weeks later, following slight bleeding or period-type pains.

The second most common cause of miscarriage is the baby not implanting itself correctly in the womb lining - another chance occurrence.

Other risk factors include the age of the mother: miscarriage risk rises as maternal age increases. For women under 35 the clinical miscarriage rate is 6.4%, at 35-40 it is 14.7%, and over 40 it is 23.1%.

Smoking, certain drugs (prescribed or illicit substances), multiple pregnancies such as twins or triplets, poorly controlled conditions such as diabetes, and auto-immune disorders such as Lupus may also increase the risk of miscarriage.” (http://www.babyloss.com/pdfs/miscarriage.pdf)