The researchers, including lead author Rodney Ritzel, Ph.D., of the University of Maryland School of Medicine in Baltimore, report their findings in the Proceedings of the National Academy of Sciences.
They suggest that the study sheds light on how the experience of pregnancy may affect brain biology to improve the chances of recovery following an injury such as a stroke.
A stroke is a serious and potentially life-threatening cardiovascular disease in which a blockage or rupture in a blood vessel cuts off the supply of oxygen-rich blood to a part of the brain.
There are two main types of stroke: ischemic strokes, which are caused by blood clots; and hemorrhagic strokes, which are caused by ruptures to blood vessels. The vast majority of strokes are ischemic and the new study confines itself to examining this type.
There is also another type of stroke called the transient ischemic attack, or mini-stroke, which is caused by a temporary blood clot. Although the clot might be temporary, the condition should still be taken seriously.
Cardiovascular diseases are the leading cause of death worldwide. In 2015, they were responsible for an estimated 17.7 million deaths, of which 6.7 million were due to stroke.
In the United States, stroke affects around 795,000 people and kills nearly 130,000 people every year.
Gender affects stroke risk
Although there are many things that people can do to reduce their risk of stroke - such as making changes to lifestyle and eating habits - age and gender are two factors that cannot be changed. The older you are, the higher your risk for stroke, especially if you are female.
The new study seeks to find out more about what influences stroke risk in women. In their report, the researchers note that while there is evidence that hormones and genetics play a role, little is known about the effects of pregnancy and giving birth.
For their investigation, the team used two groups of age-matched female mice. One group mated, became pregnant, and gave birth to multiple offspring, while the other group never mated or got pregnant.
Both groups of mice were induced to have a stroke (they underwent a 60-minute procedure that caused a reversible blockage in a blood vessel in the brain).
The researchers took measures of inflammation, volume of affected brain tissue, and behavioral recovery.
Reproduction affects stroke recovery
The results showed that the mice that had given birth multiple times showed increased risk for stroke, as evidenced by weight gain, higher blood levels of triglycerides and cholesterol, muscle fatigue, less physical activity, and suppressed immunity.
However, they also showed that, compared with those that had never been pregnant or had offspring, the female mice that had given birth multiple times showed a remarkable resistance to brain injury following stroke, as well as a much better behavioral recovery at a critical time point after the event.
The authors note, for example, that these mice "had smaller infarcts, less glial activation, and less behavioral impairment in the critical recovery window of 72 hours."
Also, at 1 month after stroke, behavioral recovery was "significantly better" in the mice that had given birth multiple times, and this recovery was accompanied by an increase in formation of new blood vessels that was tied to improvements in behavioral and mental tests.
The researchers suggest that their findings show that "reproductive experience has profound and complex effects on neurovascular health and disease," and demonstrate the importance of including females that have given birth in studies that examine the lifelong pattern of stroke risk in women.
"Mice that were pregnant and had given birth had less brain inflammation, smaller brain injuries, and recovered better after stroke, despite showing signs of increased cardiovascular risk."
Rodney Ritzel, Ph.D.