While the reason for this difference is unknown, lead researcher Peter G. Gibson, M.D., says the female fetus may produce a substance in response to the inflammation of the asthmatic mother's airways that somehow worsens the asthma.
"The good news is that the vast majority of pregnant women with asthma can control their asthma with treatment," said Dr. Gibson, Professor in the Department of Respiratory and Sleep Medicine at Hunter Medical Research Institute at John Hunter Hospital in Newcastle, Australia.
The study followed 151 pregnant women; 33 did not have asthma, 38 had asthma but did not use inhaled steroids during pregnancy, and 80 had asthma and used inhaled steroids every day during their pregnancy. Steroids control the inflammation of the airways that occurs in asthma. "Inhaled steroids are generally considered safer for the fetus than oral steroids," Dr. Gibson said.
He found that 60% of asthmatic women who were pregnant with a male fetus were symptom-free throughout their pregnancy and did not report nighttime breathing problems from the 18th to the 30th week of pregnancy. In contrast, 61% of asthmatic women pregnant with a female fetus were symptom-free at 18 weeks, but at 30 weeks only 28% were symptom-free. Nighttime symptoms increased significantly from 18 to 30 weeks in women pregnant with a female fetus.
Use of inhaled steroids increased significantly in asthmatic women pregnant with a female fetus, but did not change in asthmatic women pregnant with a male.
It is very important for a pregnant woman to treat her asthma, Dr. Gibson said. "Asthma is dangerous to the mother and baby," he explained. "If a mother has a severe asthma attack, it can result in reduced oxygen to the baby, which is needed for normal growth and development."
In a previous study published by Dr. Gibson, mothers with untreated asthma had smaller female babies than mothers whose asthma was treated with steroids during pregnancy.