What Is Preterm Labor?
You may be experiencing early or preterm labor if uterine contractions become strong enough to cause dilatation, shortening, and thinning (effacement) of your cervix. Labor is preterm if it begins prior to 37 weeks gestation, (37 weeks from the first day of your last menstrual period or LMP).
Unfortunately, waiting for uterine contractions to result in cervical changes wastes valuable treatment time, so chances are your doctor will make a diagnosis of premature labor even when your cervical changes are minor or just beginning. This is a situation when the old adage, “better safe than sorry,” really holds true.
Early treatment is more effective than later treatment (after labor has advanced). This is especially true because preterm delivery causes more death and illness for newborns than anything else except birth defects; and spontaneous preterm labor causes a large percentage of preterm deliveries.
The latest statistics on preterm births show that two percent of all deliveries will occur at less than 32 weeks (extreme prematurity); 5.5 percent between 32 and 35 weeks; and 4.2 percent at 36 to 37 weeks.
Who Is at Risk?
The risk of preterm delivery varies considerably. Some women have several times the risk of others. For example, women with a previous preterm delivery have two to three times the usual risk of having a preterm infant in a subsequent pregnancy (see following table). Some risk factors can be controlled (such as alcohol and illicit drug use), but other factors, such as age, multiple gestation, and uterine abnormalities, can’t be controlled. All women should watch for the signs of preterm labor and avoid activities that can lead to preterm labor.
Lifestyle can influence your risk of preterm labor. These are some of the few risk factors for preterm labor that you can control:
- cigarette smoking
- alcohol use
- cocaine use
- short interval between subsequent pregnancies
- poor weight gain during pregnancy
Each of these factors can increase the risk of a low birth weight baby and, probably, preterm labor and delivery. The best time to begin preterm labor prevention is before conception, yet, many pregnancies are unplanned, making preconception counseling and preparation difficult.
What Influences Preterm Labor?
Toxins in cigarette smoke pass through the placenta and travel to the baby. When you smoke, you’re sharing those toxic chemicals with your growing baby. Studies show that women who smoke have smaller babies and a higher incidence of preterm labor, placental abruption (premature separation of the placenta from the uterus), placenta previa (abnormal positioning of the placenta over the cervix) and perhaps premature rupture of the amniotic membranes.
One interesting association that has been reported by several studies is an association between psychological stress and preterm birth. The mechanism for this is not yet known. This is a risk factor that women usually can’t control, but one that appears to increase the risk of preterm birth.
The risk of delivering a low birth weight (LBW) infant is greatest at both extremes of a woman’s reproductive life. There are many factors that influence this risk. For example, young teenagers often have poor eating habits, poor weight gain, neglect to take their vitamin supplements, and fail to seek prenatal care early in pregnancy. They may also smoke, drink alcohol, or use drugs. As women get older their chance of multiple gestation (twins, triplets, etc.) increases. Multiple gestation often ends with a preterm delivery and babies that are low birth weight. Older women are also more prone to hypertension, diabetes, heart conditions , and heart and kidney disease. These diseases can complicate a woman’s pregnancy and make her more prone to preterm delivery.
Previous Preterm Delivery
Women who have delivered a premature baby have up to a 20 percent chance of having a subsequent premature delivery. If a woman has had two premature births, some researchers estimate that the risk is over 50 percent that a subsequent birth will be premature. The risk data from two landmark research studies on spontaneous and subsequent preterm births (1985 study by R.A. Carr and M.H. Hill, and the 1995 J. Kristensen study) are shown in the table below.
Risk of Premature Delivery
Subsequent PTD Chances are …
Women who have a first trimester abortion by the vacuum aspiration method seem to have no increase in their risk of ectopic (tubal) pregnancy, mid-trimester miscarriage, or preterm delivery compared with women who are experiencing their first pregnancy.
Women who have had a termination by the dilatation and curettage (D&C) method may be at increased risk for ectopic pregnancy, second-trimester miscarriage, or a preterm delivery in subsequent pregnancies.
Physical Activity and Employment
Exercise during pregnancy does not increase the risk of spontaneous abortion or preterm labor. There is evidence that regular exercise may reduce the risk of some complications of pregnancy such as gestational diabetes. A recent summary of the evidence regarding physical activity and pregnancy outcome found that regular exercise may reduce the routine discomforts and symptoms of pregnancy and decrease the length of labor. Similarly, regular exercise continued as a pattern of activity that existed before pregnancy appears harmless. Regular, low-impact exercise is encouraged if no unusual symptoms appear in response to the exercise. Some women have even run marathons during their pregnancy; however, pregnancy is not the time to begin a strenuous, new exercise regimen.
Similarly for most occupations there is little evidence that working during pregnancy causes problems in an uncomplicated pregnancy. Many women continue to work up to the time of delivery. There are reports that women who stand for long periods of time may have a higher rate of preterm delivery. Women who engage in heavy manual labor may or may not be at increased risk of preterm labor. Other reports contradict this conclusion and so the evidence is not clear. It is only common sense that women should not engage in activity that causes severe physical strain or excessive fatigue. The question of whether to work and for how long should be discussed with your doctor.
Exposure To DES (Diethylstilbestrol)
In the 1950s and 1960s, pregnant women were sometimes prescribed a synthetic estrogen hormone, DES, to reduce the risk of miscarriage. Not only was it not an effective treatment, it altered the development of female reproductive organs in the girls whose mothers took DES. Women whose mothers received DES are more likely than other women to have abnormalities of their vagina and uterus that can cause preterm labor. Females exposed to DES as a fetus have a 10 to 30 percent risk of preterm delivery due to structural problems in the uterus, cervix, or vagina and are also predisposed to vaginal cancer.
What Defines a High-Risk Pregnancy?
If you’re at risk for preterm labor your doctor may refer to your pregnancy as a high-risk pregnancy. That means your risk of complications such as premature delivery, or other problems with your pregnancy, is greater than that of the general population. Despite having a higher risk, many high-risk pregnancies do not have complications and result in normal labors and deliveries at full-term. Similarly many preterm babies are delivered to women without known risk factors. Risk assessment does not accurately predict preterm delivery; it only estimates risk, and does so imperfectly. At least 50 percent of women who develop preterm labor have no risk factors; however, if you have risk factors for preterm labor, extra precautions and vigilance give you the best chance for a healthy, full-term baby.
It may be frightening to hear that you have a higher than usual risk of preterm delivery, but being aware of the risks, you can take measures to help yourself. There are some well-known factors that increase your risk of spontaneous preterm labor and delivery. If you have any of these factors, you should discuss the situation with your doctor to learn more about your specific risk.
Many women who have preterm births have no known risk factors. Many cases of preterm labor begin for reasons that neither your doctors nor you will ever know. All pregnant women should be considered to be at risk for preterm birth and each should know the signs of premature labor.
What Are the Signs of Premature Labor?
Symptoms often (but not always) alert women to preterm labor. It is important to seek help with any of the warning signs below. A warning sign does not necessarily indicate preterm labor, but it does mean that you should contact your doctor or midwife.
Signs of Premature Labor
- Pelvic pressure: Sometimes this sensation of fullness and pressure is your only warning sign.
- Menstrual-like cramps (even if occasional)
- Watery or bloody vaginal discharge
- Pain in the lower back (usually dull and may be only occasional)
- Abdominal cramping (with or without diarrhea)
What Should You Do?
- Stop doing whatever you were doing when the symptoms began.
- Lie down on your left side.
- Drink several glasses of water.
- Feel your abdomen to see if you can feel your uterus contract (harden).
- If your symptoms get worse or do not go away within an hour, seek assistance immediately.
- If your symptoms go away, but return, seek assistance.
- Tell your doctor or midwife about the symptoms at the next visit, even if your symptoms go away when you lie down and do not return.
What You Can Do
For planned pregnancies, preconception counseling and preparation should include good nutrition and stopping the use of tobacco, illicit drugs, and alcohol. You should choose an interval of at least 18 to 23 months between a prior and succeeding birth. Assessment and referral for issues such as domestic violence and abuse are also important. All of these may lessen the risk of preterm delivery.
Preterm labor is scary, but in most cases the chances of having a healthy baby are good. Remember, lowering the risks is the key to improving your chances of a term pregnancy.