NEW YORK (Reuters Health) - Epidural analgesia administered during labor is less likely to work in women who are obese, primarily because of technical difficulties, anesthesiologists in the UK report.

Epidural analgesia involves injection of drugs through a catheter into the epidural space of the spine, blocking the transmission of pain signals through nerves in the spinal cord. This results in temporary loss of pain and sensation below the point of injection.

"Typically, a 4-inch long needle is inserted about 2.5 inches deep," lead author, Dr. Martin Dresner explained to Reuters Health. "But in very obese women, you can insert the needle up to the hilt, and you still haven't gotten to where you're going yet."

In these situations, he said, the anesthesiologist has to resort to other measures, such as using extra long needles, which is a concern because of the risk of damaging the spine. "Sometimes we have to abandon the effort, because it's not safe to continue," he added.

Dresner and colleagues audited computerized records of 12,572 women who received an epidural while in labor at Leeds General Infirmary between 1997 and 2005.

They report in the obstetric journal BJOG that nearly 42 percent of patients were overweight, 32 percent were obese, and 3.4 percent were morbidly obese.

Obstetrical nurses had been asked to assess the efficacy of epidural analgesia for their patients. Records showed a significant increase in failed epidurals as body weight increased (rising from 5.1 percent among normal-weight women, to 7.7 percent among obese women, and 11.7 percent in morbidly obese women).

Also, the c-section rate "among epidural recipients increases dramatically as body mass index rises," the researchers found, although they point out that the data did not show if administration of epidurals further increased patients' risk.

In recent years, "it has not been unusual to have patients weighing over 220 pounds or even over 300 pounds," Dresner said. He suggests that women should be weighed during pregnancy to make sure they're not gaining excess weight.

"We have to tell them that there are penalties if they gain too much weight. And remind them that obesity is not a victimless crime nor is it just a question of analgesia during labor," he said, referring to the increased risks of damage to the fetus.

Yet, the authors note that many physicians are reluctant to bring up the issue of obesity out of fear of causing offense.

Dresner emphasized that physicians providing prenatal care "need to bite the bullet and get on with this, and run the risk of maybe causing some offense. If we tell the truth gently, I don't see why we should be afraid to raise the topic."

"Frankly," he added, "if you cause a bit offense, it's a price worth paying."