Interesting article in the New York Times about an Australian study that found, among women in their study group:
Study Says Women With Mate Get Heavier
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.
Many of us have found that the crucial times for weight gain are life changes because we lose our comfortable routines, and certainly finding a partner and having a baby are huge life changes! Also, a lot of us have discovered how easy it is to have an "eating partner", when you give each other tacit permission to overeat unhealthy foods. Eating can be something that two people share together, especially two foodies. So the study's findings make sense to me.
Thinking about myself, I weighed about 170 pounds when I got married. I gained about 20 pounds when I was practicing law (stress!) and then ended up at 222 after Baby #1. After #2, I was parked solidly in the 250s and yoyo'ed up and down from that point for 18 years. So yeah, getting married and then having babies certainly were big markers for weight gain for me.