The findings, from a study of nearly 2,500 U.S. kids, add to evidence that prenatal smoking may affect children's future lung health.
There are already plenty of reasons for women to quit smoking during, and ideally before, pregnancy, said lead researcher Sam Oh, of the University of California San Francisco.
This study offers more motivation for women, and for doctors to ask moms and expectant moms about smoking, Oh said in an interview.
"Pregnancy is a great opportunity for smoking cessation," he said.
Smoking during pregnancy is linked to increased risks of miscarriage, low birth weight, certain birth defects and other pregnancy complications.
As for asthma, many studies have found that secondhand smoke may worsen children's asthma symptoms, or possibly raise their risk of developing the lung disease in the first place. The same risks have been linked to moms' prenatal smoking.
But, Oh's team says, it has not been clear how much of an impact prenatal smoking might have on kids' asthma symptoms later in life, independent of any current exposure to secondhand smoke.
HIGHER RISK AMONG POOR MINORITIES
For their study, the researchers focused on 2,481 black and Hispanic kids between the ages of 8 and 17 who all had asthma and were mostly from low-income families.
In the U.S., poor, minority children are at particular risk of asthma. About 16 percent of low-income black children have asthma, versus the national prevalence of 9 percent, according to the U.S. Centers for Disease Control and Prevention.
In this study, almost 19 percent of African-American moms smoked at some point during pregnancy, as did 5.5 percent of Hispanic moms.
Overall, their kids were at greater risk of poor asthma control later in life, even when childhood secondhand-smoke exposure was taken into account -- as well as other factors like a child's age and asthma medication use.
About 30 percent of Hispanic kids and 38 percent of black kids had poorly controlled asthma symptoms -- and the risk was 50 percent for those exposed to smoking in the womb, versus unexposed kids.