In addition, mothers' use of the drug diclectin -- prescribed in Canada for morning sickness -- did not diminish the effects. In fact, children whose mothers had used the medication showed the highest average scores on certain tests.
Together, the findings suggest that "nausea and vomiting in pregnancy is not harmful and in fact may enhance favorably children's long-term" mental development, lead researcher Dr. Irena Nulman, of the Hospital for Sick Children in Toronto, told Reuters Health in an email.
As for diclectin, she said, it "alleviates clinical symptoms of morning sickness, may improve women's quality of life during pregnancy and was not found to be associated with" side effects."
Nulman and her colleagues report the findings in the Journal of Pediatrics.
Nausea and vomiting during pregnancy is very common, particularly in the first trimester. Because it is related to changes in particular hormones that are needed for the placenta's development, one theory is that morning sickness is a sign of a healthy pregnancy.
Past studies have linked morning sickness to lower rates of miscarriage, stillbirth and preterm delivery. Whether it is related to any long-term benefits had been unclear.
The current findings are based on 45 children whose mothers had taken diclectin for morning sickness, 47 whose mothers had suffered morning sickness but not taken the drug, and 29 whose mothers had no nausea and vomiting during pregnancy.
All of the children scored within the normal range for mental development, Nulman said. But on average, certain tests scores were higher in the two groups of children whose mothers reported having morning sickness. And scores tended to climb along with the severity of the morning sickness.
It's possible, the researchers write, that the same pregnancy hormones related to morning sickness have positive effects on fetal brain development.
Morning sickness is not always benign, however. A small percentage of pregnant women progress to a condition called hyperemesis gravidarum -- severe, persistent nausea and vomiting that can lead to dehydration, malnutrition and weight loss.
No one knows what causes that disorder, and it may, Nulman noted, have different physiological underpinnings than benign morning sickness.
Did you experience morning sickness?