What causes miscarriage

Most early miscarriages (as many as 60% of first trimester ones) will remain unexplained. It is usually assumed these losses are genetic, where the chromosomes simply did not replicate correctly. Many people will assume that something that happened recently, such as an illness, fall, or exposure to something will have caused the miscarriage. This is rarely true, since by the time a miscarriage is diagnosed or begins, the baby has been lost for quite some time. Hopefully this section will help you understand the causes; you should also read the section on myths.

There are several categories of miscarriage causes:


When we talk about a hormone problem, you have likely miscarried in less than 10 weeks. After that, the placenta has taken over hormone production and any normal deficiency you have is not a factor.

Low progesterone, the most common problem, is not as easy to treat as you might hope. Progesterone suppositories, while frequently prescribed, are not proven to be helpful and often actually cause a nonviable pregnancy to last longer than it should.

The only situation where progesterone is a sure solution is with a luteal phase defect, where the corpus luteum, which is formed along with egg at ovulation, does not produce the hormones needed to sustain a pregnancy. For most women, however, this is usually not an every-month problem. Usually the situation rights itself with the next egg and the next corpus luteum. This problem, if it is a permanent one, can be diagnosed through two separate endometrial biopsies. Progesterone must be started 48 hours after ovulation to work. By the time you have missed a period, it is too late to save a pregnancy with a luteal phase defect.5

Low progesterone, however, is usually a symptom of an nonviable pregnancy, and not the cause. Doctors often prescribe progesterone suppositories out of patient pressure when the hormone levels are low, but their use is controversial and usually completely ineffective. A common treatment for a suspected progesterone problem is Clomid, a pill taken for five days early in your cycle to rev up your hormone production. Not everyone is a candidate for Clomid, and 25% of women will have decreased cervical mucus, which can actually make you less fertile. See the Sperm Meets Egg Plan for more information.

Other hormone problems may be created when you have an untreated thyroid disorder. Your thyroid function can easily be tested, and this problem is very treatable.

Chromosome Defects

There are many factors that come in to play when the egg and sperm unite and form that first cell. Even if both the egg and sperm come with perfect chromosomes, the first few cell divisions can see an abnormality crop up that would certainly be devastating. Chromosome defects that cause a newly fertilized egg to die can account for as much as 60 percent of early miscarriages.

You can usually find out if you had a baby with a chromosome problem through testing tissue from the miscarriage. This must be done RIGHT AWAY when the tissue comes out or the cells cannot grow and the test won’t work. If this is your first miscarriage, however, do not go to great lengths to save tissue. Very few doctors will test it, and a chromosomal cause for the miscarriage will be assumed without testing.

Even when you have a D&C and the doctor sends the tissue immediately, the test still might not work. (Mine didn’t.) But if you do find your baby had a chromosome defect, find a small measure of comfort in knowing that although you lost this one precious baby, the chances of it happening again are extremely small. Another threshold will be crossed, however, at age 35. At that point, your eggs will begin to age, and your odds of a chromosomal miscarriage will begin increasing dramatically.

After the 2nd trimester begins, the number of miscarriages caused by genetic factors drops to less than 10 percent.3 If you have had several miscarriages in a row, then your odds of this being your problem are quite low, about 7%.42 

Physical Problem with the Uterus or CervixSome women have a uterus that does not have the usual shape. Others have a cervix that may be weakened by a number of causes, including multiple D&C procedures or their mother taking DES when she was pregnant (although note that DES was discontinued in 1971 and most DES daughters are leaving their childbirth years behind.) Both of these problems can cause early labor, usually during a critical period from 12-24 weeks. This cause is responsible for 12 percent of miscarriages during this time period. As the baby grows, especially during the very rapid growth spurt during this time frame, the irregularly shaped uterus may not be able to expand or the weak cervix may start to open up and let the baby out. There are treatments for both of these that are quite effective—corrective surgery on the uterus and a cervical stitch that holds the cervix closed. This problem WILL REOCCUR if not treated.

A uterine abnormality often causes a miscarriage due to early labor, but it can also cause fetal demise, which is what happened to our Casey. Sixteen weeks into my pregnancy with Emily, we had an abnormal AFP screening. Since we were near the point of the pregnancy when Casey died, naturally we were frantic. We saw a perinatologist, who discovered I had a septate uterus. When I was a fetus, the two sections of tissue that normally fuse together to form the uterus only fused on the bottom. Therefore, there is a huge wall going down the middle of my uterus. When Casey implanted, he chose the middle wall. This section, however, has little blood flow. As Casey grew and required more and more blood and nutrition, this area could not support him. Although Emily chose a better implantation spot, this problem caused her to be breech and required a c-section. While I did have the surgery to correct this problem, my next pregnancy still had complications, and I was not allowed a vaginal delivery.

Be aware that there are varying levels of septums. Some are paper-thin and simply move out of the way for the baby, causing no problems. Others, like mine, increase your chance of miscarriage significantly. Only a high-level sonogram or an HSG dye test can uncover this problem.

Immune Disorders

While many experienced and well respected reproductive endocrinologists specialize in this field now, many “regular” ob/gyn doctors are quite resistant to the idea of this type of miscarriage cause and its treatment. Specialists in immune disorders claim up to an 80% success rate with women who have had three or more miscarriages, but there is still much skepticism even among infertility and reproductive specialists.

Antiphospholipid antibodies can cause blood clots in the placenta that block or slow down the baby’s blood supply, causing growth to slow or the baby to die altogether. Your blood can be tested for these antibodies. These tests are called anticardiolipins or the associated lupus anticoagulant. These are inexpensive tests, and sometimes you can get them after only one miscarriage. If antibody levels are thought to be high enough to affect the pregnancy, treatment involves baby aspirin and sometimes a blood thinner called Heparin. In rare cases, the woman is actually found to have Lupus, which may be mild enough not to affect her, but needs management anyway to protect her pregnancies (see antinuclear antibodies). While a miscarriage due to this problem can happen at any time, often the baby will grow past the first trimester. 10 to 15% of recurring miscarriages are caused by these antibodies.

Antinuclear antibodies are caused by an auto-immune problem, in Lupus or a Lupus-like syndrome, where the body attacks itself. The treatment for this problem is Prednisone, a corticosteroid, which calms down the inflammatory process of auto-immune disease. Prednisone, however, is really a horrible drug and will cause all sorts of terrible side effects, including swelling, bruise marks on the face, and discomfort. You do not ask for this drug without really needing it.6

Fetal-Blocking Antibodies work to protect the baby from the mother’s immune system, which will recognize the father’s genetic material as foreign to her body and attack it. When the sperm penetrate the egg, it provides foreign material, but it also contains histocompatibility locus antigens (HLA). The sperm’s HLA will “talk” to the mother’s HLA, which would normally attack the baby, and stimulate the mother’s body to protect the baby. In some cases, however, the father’s genetic material is too similar to the mother’s. In that case, the mother’s response is weak and insufficient to prevent her white blood cells from attacking the new cells. Standard testing for this is not yet available, and you would have be accepted into one of the few elite clinics working in this field. If your tests show you and your partner’s DNA to be too similar, you can receive injections of your partner’s white blood cells, in hopes of getting enough of his HLA in your system to stimulate a stronger protective response. This is an expensive and controversial tactic, but allegedly (a word I use since there isn’t solid 3rd party data to support it) succeeds 80% of the time.7, 42 This type of problem usually causes an early miscarriage, well before 12 weeks, and is often suspected when several miscarriages have occurred at the exact same time in the pregnancy

The average OB/Gyn may not be up to date on these immune issues. Read up on it yourself and find a specialist who can determine if this is a problem that might be affecting your babies. You are not usually a candidate for the more involved testing, which is expensive and not typically covered by insurance, until you have at least three losses.Premature Rupture of Membranes and Early Labor

Many miscarriages begin with cramping and labor-like symptoms, but true PROM and Early Labor are usually associated with babies that are in the second or third trimester. Early labor can often be treated with drugs that relax the uterus and women are placed on bed rest either at home or in the hospital.

Sometimes, however, the baby comes anyway. This is one of the most traumatic of losses, technically a stillbirth and not a miscarriage after 20 weeks, because you will hold and see your baby and beg him or her to breathe. For some women, the baby will even be born alive, but only live for a few minutes, hours or days. There really is nothing harder in life than this.

PROM is defined as your water breaking prior to 37 weeks, the age that is considered full term. Most women who have leaking or gushing amniotic fluid will be placed on antibiotics and placed in the hospital because the risk of infection is very high. Once an infection comes, the baby will almost always have to be delivered.

Babies must weigh 500 grams, or about a pound, to survive. Because I was at high risk for PROM and early labor, I kept this day on my calendar and waited with fear for it to pass. For women expecting a normal pregnancy, suddenly having your water break is very frightening. Your are stuck in the hospital, having to rely on what people tell you, and unable to get information on your own. It is scary.

PROM is thought largely to be caused by infections or inflammation of the uterus or fetal membranes. How these infections come or why they cause the membrane rupture is not completely understood. Pelvic exams and yeast infections are NOT considered to increase your risk for PROM. I do know, however, just in reality through talking with women, including a close friend of mine, that PROM tends to recur. Knowing you are at risk and taking all the appropriate precautions is essential to keeping your baby in the uterus as long as possible.

Fortunately, even though PROM cannot always be treated or prevented, most babies are able to make it far enough to survive and lead normal lives. If you have experienced unexplained PROM, I highly recommend finding a doctor with experience with this sort of pregnancy.

Others — Infections, Age, Chronic Disease

Many infections can cause miscarriage, but they are the big ones like syphilis, mycoplasma, toxoplasmosis, and malaria. An upper respiratory infection is NOT going to cause a miscarriage, even though it may worry you to death. Viruses are the same. Normal illnesses like the common cold will not cause a problem, but AIDS and German Measles can. Infections that directly affect the uterus are bigger risk. This does NOT include yeast infections, which are extremely common in pregnancy. See the section on Premature Rupture of Membranes for more information on these infections.

There are a few common illnesses that can cause a miscarriage or fetal malformation if you get them for the FIRST TIME during pregnancy, including Chicken Pox and Fifth Disease. The vast majority of women already have immunity to these diseases, however, and should not be concerned about exposure to them during pregnancy. If you think you may not have immunity, ask your doctor to run an antibody titer to see if you have a live antibody, or only an old antibody to the disease in your blood. Only the live antibody without the old antibody present is a danger.

An infection that causes a fever of over 101 degrees Fahrenheit should be treated immediately, however. There is a small risk that prolonged fever can affect your baby. Take Tylenol to keep your fever down and stay in touch with your doctor.

Age is only a factor in miscarriage when you consider what aging can do to your body. The first and most common is with chromosomes. It is not YOU who have a problem, it is likely your egg or sperm, which have also aged. Age can, however, bring other problems such as poor health, disease, or hormonal imbalance that can make a pregnancy harder to sustain. You don’t start seeing these problems in great numbers, however, until after 40.

Health problems in the mother can create problems with the pregnancy. Diabetes, heart problems, and thyroid disorders are just a few that may complicate the pregnancy. Having these does NOT mean you will certainly have a miscarriage. You will simply have to be more careful and make sure your treatments are adapted if needed during pregnancy.

Accidents typically do not cause a miscarriage. The baby is well protected in its amniotic sac, surrounded by fluid, and even a hard blow to the abdomen will likely only rock it. Most women who have a car accident, even with a certain amount of trauma, have their babies just fine.

The Unknown

The hardest thing to accept is no reason at all. You live in fear, wondering if the same terrible cause of your first baby’s death will cause another one to die. You scarcely dare to try again. I have been in this situation and I tossed my doctor’s statistics aside. I had already been on the wrong side of the statistics; I didn’t care for anymore. But I do know this. One miscarriage hardly raises your chances to miscarry again at all. You are simply back at square one. Try to put the risk as far back in your mind as possible and enjoy another pregnancy. But I understand if you can’t.

Blighted  Ovum, Ectopic Pregnancy, Molar Pregnancy, and Stillbirth

Sometimes a pregnancy ends unhappily, but it is not technically a miscarriage. This section will touch on these types of situations.

Blighted Ovum is a condition (with a terrible, unfortunate name) where the gestational sac grows, the woman gets all the pregnancy symptoms, but the baby itself never develops. The sac will continue to grow and grow, and most women do not know there is no baby until an ultrasound is done. The bleeding, if that happens before the blighted ovum is found via ultrasound, is slow and brown. Your pregnancy symptoms will seem to go away. A blighted ovum is believed to be caused by an egg or sperm with poor genetic material. When the egg is fertilized, instead of creating both a sac and a baby, the part that should be a baby never grows. A D&C is almost always needed to empty the uterus, because the body is very slow to realize there is no baby. Some women do experience more than one blighted ovum, but most women go on to later have a baby.

An Ectopic Pregnancy is a normal fertilized egg that gets stuck in the fallopian tube (although occasionally it will fall into the abdominal cavity) and implants there. This type of pregnancy cannot survive and puts the mother at great risk for severe hemorrhaging and possibly even death as the baby grows and eventually bursts the tube. When the ectopic is discovered based on pain and symptoms rather than an early ultrasound, the mother will immediately have surgery to remove the baby. Things will happen very fast, and most likely if this has happened to you, you are reading this after it is all over. If you are afraid you have an ectopic, the symptoms that you really want to watch for are: sharp, intense pain in your abdomen or possibly in your shoulder; a pregnancy test that is positive, then turns negative a few days later; and spotty red bleeding that continues day after day.  Ectopics that are caught early can be treated with a cancer drug called Methotrexate, which will end the pregnancy safely and without surgery.

Ectopics are usually caused by scar tissue in the fallopian tubes that could have been caused by: previous surgery in the pelvic region, uterus, or tubes; a pelvic infection such as chlamydia or pelvic inflammatory disease; or endometriosis that blocks the entrance to the tubes. If you have had one ectopic, your risk increases for another one. See additional information on treatment.

A Molar Pregnancy is a very rare type of pregnancy where an abnormal mass forms inside the uterus after the egg is fertilized. The baby usually does not form, but the uterus is filled with big bubble clusters.  A molar pregnancy is caused when a sperm fertilizes an empty egg (called a complete molar pregnancy) and no baby grows, or when two sperm fertilize an egg and both the baby grows a little as well as an abnormal placenta (called a partial molar.) Even if a baby does grow, it cannot survive. The longest documented molar pregnancy I have seen was a 24-week stillbirth, and most molar pregnancies will be diagnosed and a D&C performed before the end of the first trimester. If a molar pregnancy has been diagnosed, your medical condition will be carefully monitored. In about 15% of molar pregnancies (usually complete molars and not partial), the moles spread to other parts of the body like cancer. A mild form of chemotherapy will have to be used (with methotrexate), but rest assured that the cure rate for this type of disease is very high. The signs of a molar pregnancy include: bleeding in the 12th week of pregnancy, a uterus that is larger than normal, and hCG levels that are too high. The molar pregnancy is removed by a dilating the cervix and gently suctioning out the clusters. Women who have had a molar pregnancy are usually advised not to get pregnant again for at least a year to ensure the cancerous form is not present. It is absolutely essential to follow doctors orders on when to try again with a molar pregnancy diagnosis. Do not cheat, and have regular follow ups even after your hCG is zero, to make sure it does not rise again.8,9

A stillbirth is technically any pregnancy that ends after the 20th week and the baby does not survive. Some babies die in utero and are discovered when the heartbeat is not found. The most common causes of this are: uterine abnormalities, a knot or other umbilical cord accident, infections of the lining of the gestational sac or cord, and placental abruptions that cause the placenta to pull away from the uterine wall. These babies are usually born through the induction of labor, although some babies are small enough to be taken by D&C or D&E procedures.

Other babies are lost through early labor. The causes of early labor are Premature Rupture of Membranes, uterine abnormalities that make the uterus too small to hold the baby, and anincompetent cervix, which opens up and lets the baby out. Sometimes a stillbirth occurs during the birth, by an umbilical cord that gets pinched between the baby’s head and the cervix, or the cord wraps around the baby’s neck. Repeat stillbirths are extremely rare and are almost all related to uterine or cervix problems, which can be fixed or treated once found.


I know you may be frightened by what you’re going through.

Remember Deanna has a private group on Facebook to help.