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Women who fear childbirth have longer labor than those who do not, a new study has found.

Norwegian researchers assessed fear of childbirth with a 33-item questionnaire filled out at 32 weeks of gestation. After excluding those with a multiple pregnancy, breech presentation, preterm delivery or elective Caesareans, they were left with 2,206 women for the study, published online last week in the journal BJOG.

Some factors were more common in women who feared labor — for example, never having had a baby before, using drugs to speed labor, and having had a previous vaginal delivery using instruments. But even after adjusting for these and other factors, women who ranked in the upper half on the fear-of-childbirth scale had an average labor 47 minutes longer than those in the lower half. Fear of childbirth was itself an independent predictor of longer labor.

Why does this happen? “We don’t know for sure,” said the lead author, Samantha Salvesen Adams, a researcher at Akershus University Hospital in Lorenskog, Norway. “But there are two theories.

“First, stressed women have higher stress hormones during pregnancy, and high stress hormones may weaken the power of the uterus to contract. And second, we think that women who fear childbirth may communicate in different ways with health care professionals during pregnancy” — perhaps delaying measures that could speed delivery.