NHLBI/NICHD Collaborative Studies of Asthma in Pregnancy

BACKGROUND:

Asthma is one of the most common illnesses that complicates pregnancy. Asthma complicates at least 4 percent of all pregnancies; however, because at least 10 percent of the population appears to have nonspecific airway hyper-responsiveness, the true prevalence may be much higher.

Asthma can produce serious maternal and fetal complications. A number of investigators have reported an increased incidence of pre-eclampsia, gestational hypertension, hyperemesis gravidarum, vaginal hemorrhage, toxemia, and induced and complicated labor. Fetal complications include increased risk of perinatal mortality, prematurity, low birth weight, and neonatal hypoxia. In contrast, several studies have failed to confirm some or all of these previous observations. Patients with severe asthma may have the greatest risk for complications during pregnancy, in addition to the risk of maternal morbidity from the asthma. For example, it has been shown that reduced pulmonary function in asthmatic women is associated with an increased likelihood of intrauterine growth retardation. On the other hand, studies in which asthma was successfully controlled have resulted in pregnancy outcomes similar to the general population.

The mechanisms by which asthma may have adverse perinatal effects are not well known. Poor control of asthma leading to chronic or episodic fetal hypoxia is thought to be important. Medications used in asthma treatment may also play a role, although the limited data suggests minimal or no effects. In addition, it is possible that extrapulmonary autonomic nervous system abnormalities, such as uterine muscle hyperreactivity, could contribute to pre-term delivery or gestational hypertension independent of asthma control or therapy.

Unfortunately, previous studies have been limited by relatively small numbers. Few studies have controlled for factors known to affect infant birth weight, such as maternal race, height, weight, parity, nutrition, and cigarette smoking. Particularly, race may be an important contributing factor in assessing the relationship between asthma and pregnancy outcomes, since Blacks of both sexes are twice as likely to be hospitalized from asthma and three times as likely to die from asthma as whites.