After examining you, your doctor is likely to order some tests to help diagnose Preterm Premature Rupture of Membranes (PPROM) and to assess the following:
- the presence of infection;
- the degree of fetal lung development; and
- the general status of the fetus.
The most common tests for diagnosing ruptured membranes are the pH test and the Nitrazine test. To perform these tests, your doctor collects vaginal fluid during the speculum exam.
The pH test involves sampling vaginal fluid to see how acidic or alkaline it is. Normal pH for vaginal fluid is between 4.5 and 5.5, and normal pH of amniotic fluid usually falls between 7.0 and 7.5. (When measuring pH, the higher the number, the more alkaline the substance.) If a sample of your vaginal fluid is more alkaline than normal vaginal pH, then it is very likely that the membranes have ruptured and amniotic fluid has leaked into the vagina.
The Nitrazine test involves placing small amounts (a drop or two) of vaginal fluid onto paper strips prepared with Nitrazine dye. A chemical reaction occurs and the strips change color, indicating the pH of the vaginal fluid. If the color shows the pH is greater than 6.5, it's likely the membranes have ruptured. False readings can occur, however. Women with blood-tinged mucus, for example, can test positive on the Nitrazine test because blood has a pH closer to amniotic fluid than vaginal fluid. Some vaginal infections can also increase the pH of fluid in the vagina, and so can recent intercourse, because semen has a high pH.
Your doctor can also confirm that your water has broken by checking for ferning (a fern-like pattern that can be seen on microscopic exam when estrogen and amniotic fluid mix together and cause salt crystallization).
Other tests for diagnosing ruptured membranes have fallen in and out of favor among obstetricians and include:
- Measuring the levels of glucose, fructose, prolactin, alpha-fetoprotein, or diamine oxidase in the fluid thought to be amniotic fluid. High levels of these chemicals indicate that the membranes have broken.
- Staining the supposed amniotic fluid with nile blue sulfate. The resulting color will indicate whether amniotic fluid is present.
- Injecting dye into the amniotic sac (by way of a needle into a woman's abdomen). If membranes have ruptured, the dyed fluid can be seen in the vagina.
These other tests, though they work, are no more practical than the Nitrazine test and have some risk. These risks include rupture of the membranes (if this hasn't occurred already), trauma to the baby, infection, and possible adverse effects from the dye.
If your doctor suspects you or your baby may have an infection, he or she may want to confirm the diagnosis by testing amniotic fluid. This fluid must be collected by amniocentesis because amniotic fluid leaking into the vagina may be contaminated with bacteria from the vagina.
Amniocentesis can be performed successfully about half the time and carries a low (but measurable) risk of injury to the placenta, umbilical cord, and baby. Before performing amniocentesis, your doctor may order an ultrasound (an imaging technique that uses sound waves to produce a picture of the internal organs) to see how much amniotic fluid remains. In about half the cases there is not enough amniotic fluid left for testing.
Tests for fetal lung maturity have a high "false-negative rate," which means that the test may incorrectly indicate a negative result when a positive result is more accurate. If your tests show a negative result, your baby will not necessarily have lung problems; however, normal lung function is not guaranteed.
If your doctor is sure that your membranes have ruptured, it's important to determine whether your baby's lungs are mature enough to function outside the womb. This is done by testing amniotic fluid for the presence of chemicals that make up surfactants (substances in the lungs that keep the lungs from collapsing). Without the proper concentrations of these chemicals, a baby can develop respiratory distress syndrome (RDS) and may suffer from lack of oxygen.
To test for fetal lung maturity, amniotic fluid can be collected two ways: by inserting a needle through the abdomen (amniocentesis) or by collecting amniotic fluid that has leaked into the vagina (transvaginal collection). Amniocentesis carries the risks mentioned above, whereas fluid from the vagina can be collected repeatedly with little risk to you or your baby.
One test for fetal lung maturity performed on amniotic fluid is the lecithin/sphingomyelin ratio. Lecithin and sphingomyelin are substances that are present in the amniotic fluid in similar concentrations before the fetus is 34 weeks. At 34 weeks, however, the concentration of lecithin rises in relation to sphingomyelin. When the concentration of lecithin is at least two times that of sphingomyelin, the risk of respiratory distress is very low. But when it is less than two, the baby is more likely to need assistance with breathing after birth. In fact, this ratio may be more important than the weight or age of the baby in predicting the risk of respiratory distress.
Another test for fetal lung maturity involves determining the level of phosphatidylglycerol in the amniotic fluid. When this compound is found in amniotic fluid, it is a good sign that the baby's lungs are mature but it is not a definitive sign. Lack of phosphatidylglycerol, even with a normal lecithin/sphingomyelin ratio, can mean that there will be respiratory distress.
Other tests include the shake test, the surfactant-albumin ratio, fluorescent polarization, the lumadex-FSI test, and the amniotic absorbance test.
What to Expect
A Doppler ultrasound test uses sound waves to measure the speed and direction of blood as it flows through blood vessels. To measure the baby's heart rate, a technician passes a hand-held instrument over your abdomen. The instrument sends and receives sound waves, which are then amplified through a microphone. This makes it possible to hear the baby's heart beat.
If you are admitted to the hospital for PPROM and are already in labor, your doctor will evaluate how your baby is responding to your contractions. The fetal heart rate is a good indicator of fetal-well being and can be monitored through physical examination (listening for the baby's heart rate with a stethoscope) as well as by Doppler ultrasound.
A normal heart rate for a fetus is between 120 and 160 beats per minute (bpm). If the fetus' heart rate drops to below 100 bpm for about 60 seconds or so, the umbilical cord may be compressed. If the baby's heart rate drops repeatedly, your doctor may decide it is time for the baby to be delivered.
Your doctor may also order a biophysical profile (BPP) to test how well the baby is doing. The BPP uses ultrasound to see whether the baby has good muscle tone and movement, whether the baby is practicing breathing, and how much fluid is around the baby. Your doctor may also order a non-stress test, which is a recording of the baby's heart rate.
If your doctor decides to delay delivery, he or she should continue to closely monitor you and your baby to make sure that this decision remains the best course of action. PPROM may not cause a baby's heart rate to drop initially, but over time-perhaps as more amniotic fluid is lost-the heart rate may drop further. If this occurs, delivery is essential.