What causes early miscarriages at 6-8 weeks?

Early miscarriages are mostly blamed on chromosomal abnormalities in the embryo that result in the arrest of embryo growth. These are more common in women of advanced age (over 37 years).

Other reasons for early miscarriages at 6-8 weeks:

1. Hyperactive immune system - An embryo comprises genetic material from both the father and the mother, which means that genetically it is always 50% foreign to the mother’s body and her immune system. Normally, a female’s immune system is reprogrammed during pregnancy to accept this embryo. However, if your immune system is hyper-active or you have auto-immune diseases, the body may perceive the embryo as a foreign entity and the attack it. This results in embryo implantation problems, clinical pregnancies, and early miscarriages. After chromosomal abnormalities, maternal immunity problems are the most common cause of early miscarriages in women.
2. Anatomical abnormalities of the uterus - Uterine malformations can cause recurrent miscarriages if they remain undetected. A study published in the Journal Human Reproduction found that more than 65% of the women with septate uterus ended up having a miscarriage. Similarly, a bicornuate, unicornuate, didelphic, T-shaped, or tipped uterus can all cause early miscarriages. If you have large fibroids, your doctor may recommend removing them to allow for a safe pregnancy. However, not all fibroids need removal. A fibroid’s potential of causing miscarriage depends upon its location within your uterus.
3. Infections - Around 15% of all early miscarriages are caused by infections of some kind. Bacterial vaginosis (imbalance of vagina’s normal bacteria), HIV (human immunodeficiency virus), Gonorrhea, and cytomegalovirus (CMV) were all found to increase the risk of miscarriage.
4. Uncontrolled diabetes or high blood pressure - 1,228 women with at least one previous miscarriage were examined in a study, of which 797 became pregnant and 188 or about 24 percent again suffered loss of pregnancy. Even if they achieved normal blood pressure before the pregnancy, each 10 mm increase in systolic blood pressure was found to cause an 8 percent higher risk of miscarriage and each 10mm increase in diastolic blood pressure showed an 18 percent increased risk of miscarriage.
5. Thyroid disease - Hypothyroidism and raised TSH levels increase the chances of miscarriage in early pregnancy. TSH levels higher than 4.5 mU/L are associated with a greater risk of early miscarriage.
6. Blood clotting - Having genetic or hereditary thrombophilia raises your predisposition to blood clotting. Blood clots that develop in the placenta increase the risk of miscarriage and fetal loss. Some doctors advise all patients seeking IVF to test for “antiphospholipid syndrome,” so it can be managed with medications for better chances of a successful pregnancy.
7. Medications - Medicines like Misoprostol, Methotrexate, retinoids, and some NSAIDS are harmful to pregnancy and may be responsible for an early miscarriage. Never take any over-the-counter medication without your doctor’s advice during pregnancy.
8. Food poisoning - At times food poisoning is lethal for the growing embryo. Listeria, a bacterium that is sometimes found in soft, unpasteurized cheeses and other uncooked foods has been linked with miscarriage. Undercooked eggs and raw meat may cause food-borne infections such as salmonella and toxoplasma, which can result in a miscarriage.
9. Environmental causes - Excessive exposure to harmful chemicals such as Lead, Arsenic, Mercury, pesticides, etc. may raise your risk of miscarriage.

While any of the above may be responsible for an early miscarriage at 6-8 weeks, the most common reasons for miscarriages are either chromosomal abnormalities in the embryo or immunologic problems in the mother.