Women with asthma who are undergoing fertility treatment may take much longer to conceive than peers without the respiratory disorder, a small Danish study suggests.

Researchers followed 245 women with unexplained infertility who sought treatment to help them conceive. With asthma, half of them took at least 4.6 years to conceive, compared to about 2.7 years without asthma.

Several studies have linked asthma to reproduction-related problems in women, the study team writes in European Respiratory Journal, though the connection is poorly understood.

"Despite subfertility often being seen clinically in asthmatic women, a causal relationship between asthma and subfertility has never been established," lead study author Dr. Elisabeth Juul Gade of Bispebjerg University Hospital in Copenhagen told Reuters Health.

"We showed that asthma has a negative influence on fertility as it increases time to pregnancy and possibly reduces birth rate, especially above 35 years of age," Gade said by email.

While the study doesn't prove asthma causes infertility, the findings suggest that women with asthma should take steps to manage symptoms before trying to conceive and also consider starting their families at a younger age when they may not have as much difficulty getting pregnant, Gade said.

To explore the link between asthma and infertility, Gade and colleagues followed women between ages 23 and 45 who had difficulties getting pregnant, including 96 women with asthma and 149 women without the condition.

At the start of the study, the women were 36 years old on average, and generally had a healthy body weight.

The women received a variety of fertility treatments, including artificial insemination and in vitro fertilization. They were followed for at least 12 months, until a successful pregnancy, the end of treatment or the conclusion of the study.

In addition to the longer timeline to conception, women with asthma also had lower success rates. About 40 percent of the asthmatic women achieved pregnancy compared to about 60 percent of the women without asthma.

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Among women experiencing what's known as primary infertility, when they have not yet given birth to any children, about 54 percent of asthmatics and 66 percent of women without asthma conceived during the study.

With what's known as secondary infertility, when women have one child then struggle to conceive again, roughly 55 percent of the women in both groups conceived during the study.

One limitation of the study is that some conception timelines during fertility treatment relied on women to report on when they had intercourse, which can make the data less reliable, the authors note. It's also possible the results might not be representative of a broader population of women because the high cost of fertility treatments may have limited participation to more affluent women who could afford this intervention.

Asthma causes systemic inflammation in the lungs and it's possible this irritation may affect other organs and mucus surfaces of the body such as the inside of the uterus, Gade said.

More research is also needed to determine whether asthma might affect the development of ovum, or egg cells, in the early stages of reproduction, Gade said. This might lead to difficulties with implantation.

It's still unclear exactly how asthma treatment might influence the connection between this disease and infertility, Gade noted.

"We know that well-treated asthma is of importance for the pregnant woman and the unborn child, but we have not been able to show whether well-treated asthma is important for fertility," Gade said. "However, we believe it is, as a large degree of inflammatory control is present when a patient is well-treated."