Nature's cruel reality: fear of childbirth prolongs the process by up to an hour and a half. (Photo: Creative Commons: ChrisHaysPho)

It may seem cruel and a little obvious, but a new study confirms that women who fear childbirth endure longer labor. On average, excessive worry about giving birth can add an hour and half to birthing process, according to the research published in BJOG: An International Journal of Obstetrics and Gynaecology.

The Norwegian study looked at 2206 women pregnant with one child and who intended to deliver vaginally. At 32 weeks of pregnancy, each woman rated her fear of childbirth on the validated psychometric Wijma Delivery Expectancy Questionnaire, which is designed to measure fear of giving birth.

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For women who scored in the upper percentile and were categorized as having a fear of childbirth, labor lasted a full hour and a half longer than the it did for women with no such fear.

"Fear of childbirth seems to be an increasingly important issue in obstetric care. Our finding of longer duration of labour in women who fear childbirth is a new piece in the puzzle within this intersection between psychology and obstetrics," said study author Samantha Salvesen Adams.

She initially thought her team would find that extended labor could be chalked up to any number of other factors: first time mothers commonly experience longer labor anyway, or interventions like epidurals can elongate the process.

But even after accounting for these factors, fear added 47 minutes to an otherwise normal labor.

"Mental stress is associated with physiological arousal and release of stress hormones," Adams told CNN's The Chart. "During labor, high levels of stress hormones may weaken uterine [contractions]."

In other words, the adrenaline released when a body is stressed stops the oxytocin hormone production that makes a woman's uterus contract, slowing labor. It's a natural, biological response to fear, Fischbein said.

Also, women with fear of childbirth were more likely to deliver by instrumental vaginal delivery (17 percent versus 10.6 percent) and emergency caesarean delivery (10.9% versus 6.8%) as compared to women who weren't afraid of childbirth, the study says.

Poor communication between the patient and her doctor may also delay the use of interventions, and prolong labor, the researchers said, according to Mother Nature Network.

Between 5 and 20% of pregnant women have a fear of childbirth. Various factors have been associated with increased prevalence of fear of childbirth, including young maternal age, being a first-time mother, pre-existing psychological problems, lack of social support and a history of abuse or adverse obstetric events.

"There are a number of reasons why women may develop a fear of childbirth. This research shows that women with fear of childbirth are more likely to need obstetric intervention and this needs to be explored further so that obstetricians and midwives can provide the appropriate support and advice," Adams said.