Diagnosis of Preterm Labor

A pregnant woman may become aware of uterine contractions by 24 weeks of pregnancy. These contractions are normal. They are called Braxton­Hicks contractions. On occasion, Braxton-Hicks contractions are rather severe. In addition, the cervix often shortens and occasionally dilates during the early part of the third trimester. If these two events combine in a single woman, deciding whether or not she is in early labor may be extraordinarily difficult. If in fact she is not, any treatment or none at all will appear equally successful. Evaluating the treatment thus is very difficult, as the measure of success is that nothing has happened. Often, we cannot be sure that there was ever a condition that required treatment.

Despite the inexact nature of diagnosis, certain steps usually are taken when a woman complains of any of the symptoms at any time between 23 and 37 weeks of pregnancy. (There is some disagreement regarding when to initiate treatment, although generally before 23 or 24 weeks, survival of the preterm infant is unlikely. Delivery at that time is considered a miscarriage, not a preterm birth. After 37 weeks, there is no worry about prematurity of the baby.)

If you have any of the symptoms noted in the box, call your physician or midwife or go to the hospital where you will have your baby(or your “back-up” hospital if you had planned a home or birth center birth). The physicians and midwives in the practice or clinic where you have your prenatal care will have given you instructions on how to contact somebody in case of an emergency or whether to go to the hospital’s labor unit or emergency room. Signs of preterm labor can be considered an emergency.

You will most likely be advised to meet your physician or midwife at the office or hospital. First, you will have an abdominal examination to determine if the examiner can feel any contractions and to estimate the size and position of the baby. You may be placed on a fetal monitor so that a recording of contractions can be made, although this is not always necessary. The straps placed around your abdomen may be irritating to the uterus and increase the contractions, if there are any. Usually, a recording of the fetal heart tones will be made.

An examination of the cervix with a speculum may be done next. If there is any question about whether or not the membranes have ruptured, this examination will be carried out under sterile conditions. The physician or midwife will look to see any signs of cervical opening and will check to see if the membranes are ruptured. There are a variety of ways to do that. Sometimes, there is an obvious pooling of fluid in the vagina. Other times, a sterile swab can be placed into the vagina. The secretions picked up by the swab are looked at under a microscope. When amniotic fluid dries on a slide, it forms a characteristic pattern that resembles a fern. This is called ferning, and indicates that the membranes have ruptured. A type of litmus paper, called nitrazine paper, may be placed on the discharge. If the yellow paper turns blue, this shows an alkaline pH. Since the vagina is acidic, the alkalinity often is due to amniotic fluid and means that the membranes have ruptured. A number of other secretions are alkaline, however, such as blood, the vaginal discharge of certain infections, and even cervical mucus. This test, then, is not completely reliable.

During this sterile speculum examination, vaginal or cervical cultures for organisms including group B strep, gonorrhea, and chlamydia may be taken. This feels no different from a Pap smear.

If the membranes are ruptured, the physician or midwife should refrain from doing a bimanual examination-an examination with his or her fingers. When the protective membranes are torn, vaginal examinations increase the risk of infection-a danger for both mother and baby. Usually, once an examination is done, the delivery needs to occur within 24 hours. Avoiding examinations minimizes the possibility of infection and allows for more flexibility in handling the situation. This is especially important if you have ruptured membranes without signs of labor.

If the membranes have not ruptured(said to be intact), then a bimanual or digital(finger) examination allows the physician or midwife to check with more accuracy whether the cervix is effaced(thinned) or dilated(opened). Ultrasound is an alternative way of checking, used in some medical centers. If the cervix is thick and closed or just a bit open or thinned, most likely you will be observed for several hours, lying in bed on your side, and reexamined. If there is cervical change, then the diagnosis of preterm labor is made. If there is no cervical change, then you will continue to be watched or sent home, depending on whether contractions are still present. Contractions consistent with preterm labor are 5 to 8 minutes apart, or occur at a rate of 4 in 20 minutes or 8 in 60 minutes.

If the cervix is open to at least 2 centimeters(a bit more than the width of the average finger), or 80 percent or more effaced, and contractions are occurring, then the diagnosis may be made without waiting to see if the cervix changes. Under these circumstances, if treatment is delayed, the opportunity to stop the labor may be missed.

If your physician or midwife determines that you are not in preterm labor, or you become aware that your contractions have stopped, you will most likely go home with instructions to rest, to refrain from heavy work, including housework and lifting of toddlers, and to avoid vaginal intercourse, nipple or breast stimulation, and orgasm until all signs of possible preterm labor are gone or you have reached 37 weeks gestation.

If your physician or midwife determines that you are in preterm labor, then an attempt usually will be made to stop the labor if you are less than 4 centimeters dilated and less than 34 weeks gestation. After 34 weeks gestation, most babies will survive and the risks of treatment usually render such treatment inadvisable. Other reasons for not treating preterm labor are:

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Dilation of 5 centimeters or more.
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Fetal death or an anomaly known to be incompatible with life(both of which may predispose to preterm labor).
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Fetal distress or growth restriction(meaning that the baby may do better outside the uterus, despite being premature).
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Maternal bleeding.
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Maternal preeclampsia or eclampsia.
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Abruptio placentae.
* Chorioamnionitis(infection of the membranes).