More than half of all miscarriages in early pregnancy or the first trimester involve random chromosomal abnormalities. Problems with the genes or chromosomes of the baby are, by far, the most common causes of miscarriages. When a pregnancy is chromosomally abnormal, its genetic material sends out invalid instructions for embryonic growth. When this happens, the baby cannot develop the way its chromosomes are telling it to. Many times, the chromosomes in the egg split unevenly either during the egg’s first division (about the time of ovulation) or during the second division (when the egg is being fertilized). This uneven split results in too many or too few chromosomes.Occasionally, in approximately 3- 5% of all miscarriages, chromosomal abnormalities in one or both parents can lead to recurrent (repeated) miscarriages. This happens when they carry some kind of chronically defective genetic material, which is referred to as “balanced translocation” and can lead to chromosomal abnormalities in the baby and a miscarriage during the first trimester. Blood testing can identify these abnormalities. This test is called a “karyotype”, which maps out the chromosomal structure in both parents and is done by analyzing the white blood cells and the pattern they form. There is a much higher miscarriage rate for people who have this problem, but many go on to have healthy babies.

Cervical Problems

When the cervix is too weak to support the increased weight and pressure of a growing baby, it is called an incompetent cervix. A weakened cervix accounts for close to 20% of all second trimester miscarriages, which usually occurs when the baby’s weight increases. In the majority of cases, there’s no obvious cause for the cervix to be weakened. Other times, an incompetent cervix is the result of DES exposure, obstetrical trauma or second trimester abortions. An abortion at this stage forces the cervix to dilate just as much at it would have to if you gave birth, with instruments, when it’s not ready to dilate, which can lead to permanent damage to the cervix. Repeated miscarriages due to an incompetent cervix can sometimes be prevented by placing a stitch (or cerclage) around the cervix at the end of your first trimester (which is removed when you reach full-term), accompanied by bedrest.

Hormonal Causes

Progesterone is a hormone that is produced during pregnancy, which prepares the endometrium (uterine lining) for implantation of the fertilized egg and its presence is crucial for the uterus to support a growing embryo. It is believed that low levels of progesterone cause up to 40% of very early miscarriages, many undetected. If low progesterone levels are the cause, a miscarriage will usually happen before or around the time of an expected period or up to approximately 8 weeks along.

A condition called “luteal phase defect” (LPD) accounts for about 20- 25% of recurrent miscarriages. This hormonal imbalance is when women have permanently low levels of progesterone in repeated menstrual cycles, instead of just some cycles. Diagnosis is made when 2 consecutive endometrial biopsies are done in your doctor’s office, two to three days before your expected period. This is performed by taking a sample of endometrial tissue and only takes a few minutes. Treatment consists of either a medication called clomiphene citrate (or Clomid) or progesterone supplements (in the form of vaginal suppositories or injections).


There are several different infections that have been associated with increased miscarriage rates if they are newly acquired during the pregnancy. Toxoplasmosis (from infected cat feces or raw/undercooked meat), rubella (or German measles), cytomegalovirus (CMV), an initial infection of herpes (oral or genital), syphilis, chlamydia, chicken pox, bacterial vaginosis, mumps, parvovirus (or “Fifth Disease”), listeria (from contaminated food), Lyme disease (from an infected tick) or any infection that causes a high fever can be dangerous to a pregnancy.

Uterine Structure Abnormalities

About 15% of recurrent miscarriages are caused from congenital defects (which occur naturally) or diethylstilbestrol DES exposure (from the woman’s mother taking DES while pregnant with her). These miscarriages typically occur during the first trimester, but less-commonly may happen during the second trimester, as well.

Congenital abnormalities occur in varying degrees, ranging from a complete duplication of the uterus or a uterus that is completely divided to mild defects such as a minor septate uterus. A septate uterus is a fusion defect where a tissue bridge extends into the uterus at the point where the fusion didn’t occur properly. It may act as a partial wall, dividing the uterus. Women with DES malformations usually have an under-developed, abnormally small uterus (and/or cervix) and under-development can’t be corrected by surgery. Most women with a congenital defect do well in pregnancy and have a high success rate of carrying a healthy baby to term once the malformation has been surgically corrected.

Benign tumors called “fibroids” sometimes develop in (or on the surface of) the uterus and may cause miscarriage if they are large and interfere with implantation or limit space for the growing baby. Many times, fibroids don’t cause miscarriages and the majority of women with fibroids conceive and carry a baby to term without any problems.

Maternal Illness

Health problems in the mother such as preexisting diabetes, thyroid disease, very high blood pressure and lupus (or systemic lupus erythematosus) can lead to miscarriages. Diabetes will only cause a miscarriage if it’s late-stage or very poorly controlled and gestational diabetes (diabetes that develops during pregnancy) does not put a woman at an increased risk of miscarriage. If diabetes isn’t properly controlled during pregnancy, it not only can cause a miscarriage, but can cause many health problems for mom and baby.

Thyroid disease accounts for approximately 5% or less of recurrent miscarriage and usually only causes miscarriages prior to the tenth week of pregnancy. There are two types of thyroid disease: hypothyroidism (when your thyroid doesn’t produce enough thyroid hormone) and hyperthyroidism (when it produces too much thyroid hormone). Only hypothyroidism has been clearly linked with miscarriage. To treat hypothyroidism, you would take a thyroid replacement hormone, in the form of a pill.

A condition called “antiphospholipid antibody syndrome”, which about one-third of lupus patients posses, accounts for up to 15% of recurrent miscarriages. This can lead to blood clots forming in the blood vessels of the placenta, which may deprive the baby of oxygen and nutrients. If mild, they can cause growth retardation, but if severe, they can lead to miscarriage. A baby aspirin a day, possibly in addition to a stronger blood thinner such as heparin may be prescribed by your doctor to prevent further miscarriages.

Lifestyle and Environmental Factors

A woman’s lifestyle increases her risk for miscarriage. Smoking cigarettes, consuming a large amount of caffeine, drinking alcohol and using drugs have all been shown to increase the risk of miscarriage dramatically. Generally, the more a woman smokes, drinks alcohol, consumes caffeine and uses drugs (especially cocaine)- the more her risk of miscarriage increases. Pesticides, high levels of radiation, formaldehyde, lead and certain industrial solvents can also cause miscarriage. There are numerous medications that need to be avoided during pregnancy and some common pain-relieving drugs have been associated with the increased risk of miscarriage when taken around the time of conception. Talk with your doctor before taking any medications when you are pregnant or trying to conceive.

Miscarriage Symptoms
Causes of Miscarriage
Fathers Coping with Miscarriage
Miscarriage and Children