"After adjusting for other variables, the 10-year weight gain for an average 140-pound woman was 20 pounds if she had a baby and a partner, 15 if she had a partner but no baby, and only 11 pounds if she was childless with no partner...There is no reason to believe that having a partner causes metabolic changes, so the weight gain among childless women with partners was almost surely caused by altered behavior. Moreover, there was a steady weight gain among all women over the 10 years of the study."One of their suppositions was that with a partner comes a circle of friends; socializing leads to eating out at restaurants with fatty foods and large portions. Hm, possibly contributing. I have a different take on it.
Factor 1: Other people in your life demand your time. This is a good thing, and goes both ways. When I was single and on into early stages of dating, I had a membership at an awesome gym. I went to the gym for hours, several times a week; I love studio classes. When I wasn't at the gym, I was going on long, challenging bike rides, training for biking events, or going for local/regional hikes. One of my partner's biggest complaints, early on, was that I was spending too much time at the gym and not enough with him. In the interest of compromise and making the relationship work, I significantly cut back on my workouts for my spouse. When children are added to the family, your personal time is even more popular with them.
Factor 2: Caring for children and eating their food doesn't burn calories. After having kids, sure you gain lots of weight during pregnancy and trying to get rid of it seems simple: go to the gym! But, wait. Who will watch the child while you go to the gym? Will you entrust your newborn to the gym nursery that never gets cleaned? Do you even have the cash for the gym membership, or has your diaper budget taken all of your thin margin? Also, with small children, you're basically doing a lot of cooking, cleaning, and sitting around entertaining them. Not much cardio. AND, once kids start eating solid food, they refuse to eat healthy things! You wind up stocking more and more junk food in the house to keep them fed, and start eating it yourself. I was reading somewhere that a major cause of weight gain in moms was during meal clean-up, eating the food your picky kids leave on their plates, rather than toss or save as leftovers. Don't get in that habit.
Factor 3: There is no more spare time. Available time is an issue. I work full-time, I am married, I have two kids, hobbies, friends, and responsibilities. Finding hours or minutes to fit in cardiovascular activities, indoors or out is tough.
All said, it's tough to avoid weight gain with major lifestyle changes. When you're single, you can be as active as you want; you're not accountable to anyone but yourself. Is weight-gain when partnered metabolic? Probably not. I think that when you make compromises in your lifestyle to accommodate other people, you might gain weight without realizing it. If you want to take the weight off, it takes awareness of the changes that occurred and an effort to be as active as you were before, or to eat less of the kid-friendly foods and find healthier options for you. Fitting cardio activities back in wherever possible (I changed my schedule to work out on the lunch hour) can really help, too. Best case: find an activity that your collective family enjoys and can do together!