fetal scalp blood sampling.
Fetal scalp blood sampling is performed to measure the pH level of a baby’s blood and determine whether the baby is receiving enough oxygen from the mother during labor. The procedure is typically suggested in cases where fetal distress is suspected, as indicated by tracings from an electronic fetal monitor.
How the Procedure Is Performed
Fetal scalp blood sampling requires that the amniotic sac be ruptured. If it has not broken naturally during labor, the physician will rupture it artificially before the procedure can begin.
To collect the sample, the mother is first cleansed and the presenting part of the baby is determined. A sterile amnioscope is inserted into the vagina so that the presenting part of the baby—usually the head—can be visualized using a light source.
At the onset of a contraction, a small puncture wound is made on the baby’s scalp. When obtaining blood from the baby’s head, the fontanelles (the soft spots on the skull) must be carefully avoided.
A specialized lancet is used to create a small incision designed to penetrate approximately 2–3 millimeters in depth, with the width of the incision not exceeding 2 millimeters. A drop of blood is collected using a capillary tube, mixed, and analyzed immediately.¹
After the blood sample is obtained, the incision site must be carefully monitored until all bleeding has stopped. The baby must also be observed after birth and until discharge from the hospital, as infection can occur at the sampling site.
Limitations of the Procedure
The usefulness of fetal scalp blood sampling is limited by several factors.²
For example, the procedure cannot be used in mothers with HIV or hepatitis because infection could be transmitted through the open wound created on the baby’s scalp.
It can also be difficult to perform if:
The cervix is positioned posteriorly
Labor is in an early stage with little cervical dilation
The baby’s head is still high in the pelvis
Additionally, the procedure is subject to error by both the technician performing the sampling and the equipment used to analyze the blood.
Only the pH level of the baby’s blood is measured, and many variables can influence that value. Factors that may affect the pH measurement include:
Formation of a caput (swelling) on the baby’s scalp
Pressure exerted by the baby’s head as it opens the cervix during the second stage of labor³
The mother’s posture during labor
Medications administered to the mother
Hyperventilation
Uterine contractions⁴ ⁵
Because these factors can influence the results, the pH measurement may not always accurately reflect the baby’s true oxygen status.
Potential Benefits and Risks
Fetal scalp blood sampling can sometimes reduce the rate of cesarean deliveries. If the sample shows that the baby’s oxygen levels are adequate, labor may be allowed to continue rather than proceeding immediately to surgery.
However, this potential benefit must be weighed against the risks associated with the procedure.
Possible complications include:
Hemorrhage
Infection
Breakage of the blade used to obtain the blood sample
Trauma to the baby’s scalp
In one report involving 640 cases, two hemorrhages were documented—one resulting in stillbirth and the other in neonatal death.⁶
Another documented case required an emergency cesarean section due to continued bleeding from the fetal scalp sampling site despite the application of pressure.⁷
When the baby was delivered, the Apgar scores were 2 at both one and five minutes after birth. The baby required one stitch to close the scalp wound and also required a blood transfusion.
Although such events are extremely rare, they demonstrate the potential seriousness of complications associated with the procedure.
There has also been at least one reported case in which a fetal scalp sampling blade broke during the procedure and fragments were later identified in the baby’s scalp on an X-ray.⁸ Due to the difficulty of removal, the fragments were left in place. The infant recovered and progressed well.
A Simpler Alternative: Scalp Stimulation
A truly viable and noninvasive alternative to fetal scalp blood sampling is simple stimulation of the baby’s scalp using the examiner’s fingers.
Scalp stimulation can be just as reliable in predicting fetal well-being as blood sampling techniques.⁹ This method is especially useful when electronic fetal monitor tracings show non-reassuring heart rate patterns.
The use of scalp stimulation could reduce the need for fetal blood sampling by as much as 54 percent. When finger stimulation is used to assess fetal heart rate variability before introducing blood sampling, the reduction may reach as high as 73 percent.¹⁰
Scalp stimulation can also reduce the need for other invasive procedures by providing reassurance that the baby is tolerating labor well.
Vibroacoustic Stimulation
Another alternative to fetal scalp blood sampling is vibroacoustic stimulation (VAS).
This technique uses a brief stimulus—typically lasting three to five seconds—to awaken the baby using sound and vibration. Babies who respond with movement following the stimulus are considered unlikely to require further invasive testing.
If the baby does not respond, additional evaluation may be necessary, which could include procedures such as fetal scalp blood sampling.
Studies have shown that vibroacoustic stimulation can prevent unnecessary intervention caused by falsely abnormal electronic monitor tracings, improving the overall outcome of birth.¹¹
Questioning the Need for Fetal Scalp Blood Sampling
Several studies have questioned the practicality of fetal scalp pH sampling.¹² ¹³
Researchers have cited multiple concerns, including:
The difficulty of obtaining accurate and timely samples
The cost of specialized equipment
The limited correlation between pH alterations and actual neurological outcomes in newborns
In one large obstetrical service, fetal scalp blood sampling was nearly eliminated over a seven-year period. Notably, there was no increase in cesarean deliveries for fetal distress and no increase in complications among newborns.¹⁴
These findings raise important questions about the true usefulness of fetal scalp blood sampling in determining fetal stress during labor.
Rethinking the Standard of Care
Based on these observations, some researchers conclude that there is little need for fetal scalp blood sampling in the diagnosis and management of suspected fetal distress during labor.¹⁵ ¹⁶
According to one author, the primary beneficiary of this procedure may be the physician rather than the mother or baby. As a result, some argue that fetal scalp blood sampling should no longer be considered a routine component of modern obstetric care.¹⁷