People who were underweight (BMI less than 18.5) also experienced worse outcomes from COVID-19.
Fitness trackers and step counters seem to motivate users to meet their physical activity goals.
The new investigation is believed to be the first to measure changes in fetal brain activity in the womb, and as early as six months into pregnancy.
Western countries such as the US and the UK have seen some of the highest death rates from COVID-19 in the world, which are likely to correlate with unhealthy lifestyles.
Excess weight causes around 1,900 more cases of bowel cancer than smoking in the UK each year, the charity said in a statement.