NEW YORK (Reuters Health) – Performing fetal blood sampling (FBS) in women in early labor (cervical dilation to 3 centimeters or less) or performing multiple FBS procedures may modestly increase the risk of delivery by cesarean section, a new study from the United Kingdom suggests.

Nonetheless, Dr. Alexander Heazell, from the Maternal and Fetal Health Research Centre, University of Manchester, thinks doctors “should do this test because it avoids unnecessary Caesarean sections.”

Lowering C-section rates is a “contentious issue in the UK (and other high-income countries),” Dr. Heazell noted in an email to Reuters Health. “The way we currently monitor babies well-being, by analysis of babies’ fetal heart rate, has insufficient accuracy to identify babies at risk,” he explained.

“Importantly,” he noted, even when a baby has a “pathological” heart rate trace, only half are really in distress (short of oxygen). Therefore, obstetricians often take a fetal blood sample to measure the babies’ oxygen level. “When a woman is in early labor, obstetricians might be reluctant to do this test, as it is difficult (and) they believe the woman is likely to need a Caesarean section anyway,” Dr. Heazell noted.

Complicating the matter, published data to help guide clinicians regarding the likely outcome of labor when an FBS is required in early labor – particularly when labor is likely to go on for several hours and repeat FBS may be needed – are lacking, he and his colleagues note in the March 15 issue of the obstetrics journal BJOG.

To investigate further, they analyzed 381 consecutive women who had between one and six FBS procedures during labor at secondary and tertiary obstetric units in the UK.

After adjusting for factors that could affect the likelihood of cesarean section (including parity, previous C-section, BMI, gestation, induction/augmentation, and maternal age), women who had their first FBS in early labor had a modest increased risk of cesarean delivery (odds ratio, 1.80), compared to women who had their first FBS during established labor (cervical dilation 4 cm or more).

C-section was “only slightly” more common in the 147 women who needed more than one fetal blood sample (OR, 1.71), Dr. Heazell noted. Nearly half of the women who had a FBS procedure in early labor proceeded to vaginal delivery. Therefore, when contemplating FBS in early labor, clinicians “should not be put off by the perceived low chance of vaginal delivery, but repeating FBS on more than three occasions should be considered carefully,” Dr. Heazell and colleagues say.

In terms of fetal morbidity, the researchers report that 23 babies had to be admitted to the neonatal intensive care unit (NICU). There was no increased likelihood for NICU admission for the first FBS performed in early labor (OR, 1.11). However, babies who had three or more FBS procedures had a greater than twofold increased likelihood of being admitted to NICU (odds ratio, 2.69). The reasons for this are unclear and require further study, the investigators emphasize in their report.

They note that the proportion of babies delivered for acidaemia increased with every FBS, yet the odds ratio for fetal acidaemia among babies born to women who had three or more FBS, compared with those who had two or less, was not significant (OR, 1.59). The investigators urge caution in interpreting this finding because a significant number of missing cord pH values and few admissions to the NICU.

They conclude: “Given the increased rate of cesarean section in women requiring two or more FBS, and the modest increased rate of NICU admission in this group, it is prudent to involve a senior obstetrician in the care of these women.”

Based on this study, Dr. Heazell thinks that if a woman has an abnormal fetal heart rate in early labor, “it is worth trying to get a fetal blood sample, as the chances of Cesarean section are only slightly higher than for a woman needing fetal blood sampling in established labor.”