Preterm labor is when you go into labor before you are 37 weeks pregnant. Labor is defined as having uterine contractions with dilation of the cervix. 1 in 10 women will experience preterm labor.
Possible early signs of preterm labor are frequent uterine contractions (more than 4 per hour), pressure in your pelvis, increased vaginal discharge, vaginal spotting, and/or constant back pain.
Common causes of preterm labor are stress, genital infections, bladder and kidney infections, uterine abnormalities, placental abruption, and having a twin pregnancy. If you have a history of having preterm labor in a prior pregnancy or if your mother experienced it, you may have a higher chance of having it as well.
If you are at a high risk for preterm labor, your health care provider will often see you more frequently and may give you medications to help prevent it from happening.
If you have had a preterm delivery and you are pregnant with only one baby, there is good evidence suggesting that taking weekly intramuscular progesterone (250 mg of hydroxyprogesterone caproate) will reduce your risk of having a preterm delivery by 1/3. The injections are started at 16 weeks gestation and continued until you are 36 weeks.
Some studies have noted a significant reduction in preterm birth as well in women with a short cervix (length of 20 mm or less per ultrasound) who took daily vaginal progesterone. A common dose used for this purpose is 100 to 200 mg per day and is continued until 36 weeks gestation.
If you are at a risk of having preterm labor and you are between 24 and 34 weeks gestation you might be offered a test called a “fetal fibronectin”. This involves obtaining a sample of vaginal secretions from the upper vaginal region using a small cotton swab. If the test shows the presence of a specific protein called fibronectin, you may have a higher chance of having preterm labor over the next 2 weeks. On the other hand, if the protein is not detected, your chance of going into preterm labor over the next 2 weeks is very small. Fewer than 5 women out of a 100 with a negative test will go into labor over the next 2 weeks.
Using a vaginal probe ultrasound to measure the length of the cervix can also help your doctor detect early signs of preterm labor. Cervical length of greater than 2.5 cm (1 in) is reassuring and preterm delivery is less likely to occur.
If you develop preterm labor, bed rest might be suggested even if more recent studies have not shown it to be very useful. Medications might also be prescribed to relax your uterus. These medications are called tocolytics.
The most common oral tocolytic used in the United States is Nifedipine (Procardia®). Its side effects include heartburn, headaches and constipation.
Oral Indomethacin (Indocin®) is also often used. Its side effects include heartburn, decreased amniotic fluid, and changes in the baby’s heart rate. Fortunately, the changes in the baby’s heart rate and the reduction in amniotic fluid are reversible. For these reasons, Indomethacin is used very carefully and for very short times only. It is often not used after the beginning of the third trimester.
Another common medication is Terbutaline (Brethine®). It can be given as an isolated intramuscular shot or as a pill. Side effects include feeling a little jittery and an accelerated pulse.
Magnesium sulfate is the most common intravenous tocolytic drug for preventing preterm labor and is used as a last resort. It is only given in the hospital. It will relax all of your muscles in your body, in addition to your uterine muscles, so you will feel rather weak. It will often also relax the eye muscles, so you may notice blurred vision when on it. It is a very strong medicine, so you are watched very carefully in the hospital when you are receiving it.