The elation that accompanies the news that you're expecting can sometimes be tempered by fears about miscarriage—especially if you've experienced an earlier pregnancy loss. Although miscarriages are common, affecting around 20 percent of pregnancies, the risk decreases significantly once a woman reaches her second trimester, provided her pregnancy progresses without complication.
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First-Trimester Risk
Most miscarriages are caused by chromosomal abnormalities that occur during the first 12 weeks of pregnancy, states Dr. Linda Burke-Galloway, a board-certified ob/gyn from Florida and author of "The Smart Mother's Guide to a Better Pregnancy."
"Most miscarriages occur randomly shortly after fertilization due to an abnormal number of chromosomes during the time of fertilization," Burke-Galloway explains. "An embryo with an abnormal number of chromosomes usually does not survive. This is why most miscarriages occur during the very early part of the pregnancy."
An additional 15 percent of miscarriages occur before a pregnancy can be clinically recognized on an ultrasound, she notes. These "chemical pregnancies," which can only be confirmed through a blood test, usually end within a few weeks of conception.
Second-Trimester Risk
Miscarriage during the second trimester, which includes weeks 13 to 27 of pregnancy, is rare. Miscarriage affects just 1 to 5 percent of pregnancies between the 13th and 19th weeks of pregnancy, according to a 2007 study in American Family Physician.
After 20 weeks, a miscarriage is considered a stillbirth, but this is also rare: Just 0.3 percent of pregnancies will end in stillbirth between weeks 20 and 27, according to the same study. Miscarriages during the second trimester typically occur from anatomic abnormalities, Burke-Galloway says.
"A miscarriage could occur later during the pregnancy if the patient has an incompetent cervix, meaning the cervix is abnormal," she explains. An incompetent cervix is a second-trimester pregnancy complication that usually happens as the fetus grows in the uterus. In these cases, a procedure known as cerclage, in which the cervix is sutured closed, can be used to prevent pregnancy loss or premature birth.
Other unseen factors, such as placental abruption or a septate uterus, can contribute to second-trimester miscarriage. A placental abruption occurs when the placenta peels away from the uterus, and a septate uterus features a thin membrane that separates the organ. Finally, underlying uncontrolled chronic health conditions such as diabetes or high blood pressure can increase a woman's chance for miscarriage during the second trimester.
Third-Trimester Risk
As with second-trimester miscarriages, stillbirths that occur during the third trimester, or between weeks 28 and 40 of a pregnancy, are rare, accounting for less than 1 percent of all miscarriages, according to the National Institutes of Health. Therefore, by the time your pregnancy has progressed to the third trimester, your chance of delivering a healthy, full-term baby after week 37 is more than 99 percent.
Most stillbirths occur because of unexpected complications, such as the premature rupture of the amniotic sac or placental abruption, which occurs when the amniotic sac separates from the wall of the uterus. Complications from the umbilical cord, a sudden maternal illness or infection with a high fever, or the uncontrolled management of chronic conditions like diabetes or high blood pressure can also contribute to later-term pregnancy loss.
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Throughout the Pregnancy
No matter what stage of pregnancy you're in, proper prenatal care is essential, especially if you are managing a chronic health condition such as diabetes or a thyroid disorder while you're expecting. If you've experienced recurrent pregnancy loss, particularly in your second or third trimester, your health care provider likely will want to order further testing to rule out any genetic or undiagnosed diseases that could prevent you from carrying future pregnancies to term.
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