Going to university, getting a job and becoming a parent are key life events for piling on the pounds, experts say.
Academics at the University of Cambridge analysed dozens of studies to find out the key times people put on weight - and noted that exercise levels often drop at the same time.
The first set of six studies looked at weight gain in mothers compared with women who remained childless. A woman of average height (5ft 3in) who had no children gained around 7.5kg (16.5lb) over five to six years, while a mother of the same height put that on plus an extra 1.3kg (2.9lb).
One study looked at the impact of becoming a father and found no difference in change in body mass index.
Evidence on exercise showed that mothers and fathers tended to do less exercise than childless people. Writing in Obesity Reviews, the experts said: "Becoming a mother is associated with 17% greater absolute BMI (body mass index) gain than remaining childless.
"Motherhood BMI gain is additional to an alarming BMI increase among young women, highlighting the need for obesity prevention among all young women, including new mothers."
Nine studies found that leaving high school was associated with a decrease of seven minutes of exercise. The impact on men was bigger than on women.
The change was biggest when people went to university, with overall levels of moderate to vigorous physical activity falling by 11.4 minutes per day.
Three studies also showed people put on weight after leaving school, while others found decreases in physical activity on starting employment.
Tam Fry, from the National Obesity Forum, said: "UK Governments have never given secondary school PE the commitment it deserves. So, apart from the few who exercise in all weathers, physical activity as a daily routine becomes a thing of the past for millions as they grow into young adults."