Scalp pH testing

Alternative names
Fetal scalp blood; Fetal blood testing

Definition

Fetal blood testing is a transvaginal (into the vagina) procedure performed during active labor. The scalp of the fetus is cleansed and pierced, and a small blood sample is taken for evaluation.

How the test is performed

The procedure typically takes about 5 minutes. The mother is placed in the lithotomy position (on her back with knees and legs pulled toward the chest). If her cervix is dilated at least 3 to 4 centimeters, a plastic cone is placed in the vagina and fit snuggly against the scalp of the fetus.

The scalp is cleansed and dried with long cotton swabs, and a small amount of petroleum jelly is applied so the blood droplets will form beads for easier collection. A small puncture is made in the scalp and blood droplets are collected in a thin tube. The tube is either sent to the hospital laboratory or analyzed by a machine in the labor and delivery department. In either case, results are available in just a few minutes.

If the woman is unable to be still, it will be very difficult to perform this test. If her cervix is not dilated enough, it is impossible to perform this test.

How to prepare for the test
The health care provider will explain the procedure and its risks. There isn’t always a separate consent form for this procedure because many hospitals consider it part of the general consent form you signed at admission. The only preparation required is to remain calm, because if the mother is upset it might be difficult to perform the test.

How the test will feel

The procedure should feel like a long pelvic exam. At this stage of labor, many patients already have an epidural in place and may not feel the procedure at all.

Why the test is performed

Usually this test is performed to obtain information about fetal acid-base balance (blood pH). Sometimes fetal heart monitoring doesn’t provide enough information about the well-being of a baby. In these cases, testing the scalp pH can help the doctor decide whether the fetus is getting enough oxygen during labor. This helps determine whether the baby is healthy enough to continue labor, or if a forceps delivery or cesarean section might be the best route of delivery.

Although the test is not uncommon, most deliveries do not involve fetal scalp pH testing.

Normal Values

     
  • Normal pH: 7.25 - 7.35  
  • Borderline pH: 7.20 - 7.25  
  • Low pH: less than 7.20

What abnormal results mean

In general, low pH suggests that the baby is not tolerating labor very well. However, the results of a fetal scalp pH sample need to be interpreted in the context of each individual delivery. The doctor should explain the test results. The doctor may find the results reassuring, and labor may continue as before. Alternatively, he or she may feel that the results indicate that the baby needs to be delivered quickly, either by forceps or by cesarean section.

The results may not really point in either direction, and the test may need to be repeated in 20-30 minutes. It may need to be repeated a few times during a complicated labor.

Low pH values in cord blood taken from an umbilical cord artery can also suggest that the fetus is not getting enough oxygen during labor. Because scalp pH testing is only one of many tests by which the baby may be evaluated during labor, the results must be interpreted in context and explained to the mother.

What the risks are
Risks include the following:

     
  • Continued bleeding from the puncture site (more likely if the fetus has a pH imbalance)  
  • Infection  
  • Bruising of the baby’s scalp

Special considerations

This test may not be advised for HIV-positive mothers, or mothers who carry the hepatitis C virus. In these cases, the procedure may increase the risk of transmitting infection from mother to baby.

Johns Hopkins patient information

Last revised: December 7, 2012
by Sharon M. Smith, M.D.

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