What is a miscarriage?
Miscarriage is the loss of a pregnancy in the first 20 weeks. (In medical articles, you may see the term "spontaneous abortion" used in place of miscarriage.) About 10 to 20 percent of known pregnancies end in miscarriage, and more than 80 percent of these losses happen before 12 weeks.
This doesn't include situations in which you lose a fertilized egg before a pregnancy becomes established. Studies have found that 30 to 50 percent of fertilized eggs are lost before or during the process of implantation – often so early that a woman goes on to get her period at about the expected time.
What causes a miscarriage?
Between 50 and 70 percent of first-trimester miscarriages are thought to be random events caused by chromosomal abnormalities in the fertilized egg. Most often, this means that the egg or sperm had the wrong number of chromosomes, and as a result, the fertilized egg can't develop normally.
Sometimes a miscarriage is caused by problems that occur during the delicate process of early development. This would include an egg that doesn't implant properly in the uterus or an embryo with structural defects that prevent it from developing.
Since most healthcare practitioners won't do a full-scale workup of a healthy woman after a single miscarriage, it's usually impossible to tell why the pregnancy was lost. And even when a detailed evaluation is performed – after you've had two or three consecutive miscarriages, for instance – the cause still remains unknown half the time.
When the fertilized egg has chromosomal problems, you may end up with what's sometimes called a blighted ovum (now usually referred to in medical circles as an early pregnancy failure). In this case, the fertilized egg implants in the uterus and the placenta and gestational sac begin to develop, but the resulting embryo either stops developing very early or doesn't form at all.
Because the placenta begins to secrete hormones, you'll get a positive pregnancy test and may have early pregnancy symptoms, but an ultrasound will show an empty gestational sac. In other cases, the embryo does develop for a little while but has abnormalities that make survival impossible, and development stops before the heart starts beating.
If your baby has a normal heartbeat – usually first visible on ultrasound at around 6 weeks – and you have no symptoms like bleeding or cramping, your odds of having a miscarriage drop significantly and continue to decrease with each passing week.
What kinds of things might put me at a higher risk for miscarriage?
Though any woman can miscarry, some are more likely to miscarry than others. Here are some risk factors:
- Age: Older women are more likely to conceive babies with chromosomal abnormalities and to miscarry them as a result. In fact, 40-year-olds are about twice as likely to miscarry as 20-year-olds. Your risk of miscarriage also rises with each child you bear.
- A history of miscarriages: Women who have had two or more miscarriages in a row are more likely than other women to miscarry again.
- Chronic diseases or disorders: Poorly controlled diabetes and certain inherited blood clotting disorders, autoimmune disorders (such as antiphospholipid syndrome or 10011584 lupus), and hormonal disorders (such as polycystic ovary syndrome) are some of the conditions that could increase the risk of miscarriage.
- Uterine or cervical problems: Having certain congenital uterine abnormalities, severe uterine adhesions (bands of scar tissue), or a week or abnormally short cervix (known as cervical insufficiency) up the odds for a miscarriage. The link between one type of uterine fibroid (a common, benign growth) and miscarriage is controversial, but most fibroids don't cause problems.
- A history of birth defects or genetic problems: If you, your partner, or family members have a genetic abnormality, have had one identified in a previous pregnancy, or have given birth to a child with a birth defect, you're at higher risk for miscarriage.
- Infections: Research has shown a somewhat higher risk for miscarriage if you have listeria, mumps, rubella, measles, cytomegalovirus, parvovirus, gonorrhea, HIV, and certain other infections.
- Smoking, drinking, and using drugs: Smoking, drinking alcohol, and using drugs like cocaine and MDMA (ecstasy) during pregnancy can all increase your risk for miscarriage. Some studies show an association between caffeine consumption and a higher risk of miscarriage.
- Medications: Some medications have been linked to increased risk of miscarriage, so it's important to ask your caregiver about the safety of any medications you're taking, even while you're trying to conceive. This goes for prescription and over-the-counter drugs, including nonsteroidal anti-inflammatory drugs (NSAIDs) like ibuprofen and aspirin.
- Environmental toxins: Environmental factors that might increase your risk include lead; arsenic; some chemicals, like formaldehyde, benzene, and ethylene oxide; and large doses of radiation or anesthetic gases.
- Paternal factors: Little is known about how the father's condition contributes to a couple's risk for miscarriage, though the risk does rise with the father's age. Researchers are studying the extent to which sperm could be damaged by environmental toxins but still manage to fertilize an egg. Some studies have found a greater risk of miscarriage when the father has been exposed to mercury, lead, and some industrial chemicals and pesticides.
- Obesity: Some studies show a link between obesity and miscarriage.
- Diagnostic procedures: There's a small increased risk of miscarriage after chorionic villus sampling and amniocenetesis, which may performed for diagnostic genetic testing.
Your risk of miscarriage is also higher if you get pregnant within three months after giving birth.
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