Weight after a birth is a distribution, not a target
In a US cohort of 774 women who were actually weighed, the average retained at one year was 11.1 lb, with a standard deviation of 21 lb. The average describes almost nobody.
This topic exists to take a target away from you, not to give you one. Nothing in it is a goal and nothing in it is a failure. Read it that way or skip it.
An American study followed 774 women at five sites and weighed and measured them at six and twelve months rather than asking them. That matters, because self-reported weights are unreliable in exactly the direction you would expect. The women were mostly on low incomes: 46% African American, 27.8% Hispanic, 24.7% white, and 39.4% living below twice the federal poverty level. Their average weight before pregnancy was 161.5 lb, with a range from 82 to 345 lb.
Here is the average retention, and here is why the average is nearly useless.
| Weight held above pre-pregnancy weight | Average | Standard deviation |
|---|---|---|
| At 6 months | 12.0 lb | 20.3 lb |
| At 12 months | 11.1 lb | 21.0 lb |
The standard deviation is a measure of spread. Here it is roughly twice the average itself. In plain terms: the women in this study were scattered so widely that "about eleven pounds" is not a description of any of them. It is an arithmetic artefact of adding together women who had lost weight and women who had gained a great deal.
So use the distribution instead. At one year, out of 774 women:
Which leaves 25.2%, about 195 women, at or below their pre-pregnancy weight at one year. That last figure is calculated by this guide as the remainder of the 74.8%; the paper prints the 74.8%.
The authors also state, in their own words, that approximately one in six women who started the pregnancy with a normal BMI also retained more than 20 lbs weight at 1 year postpartum.
What actually predicted where a woman landed. In the authors' models, the things that mattered were her weight before the pregnancy, how much she gained during it, and her race and her poverty level. Holding 20 lb or more at a year happened to 24.2% of these women overall. Each extra pound gained in pregnancy above the national recommendation raised the odds of that by about 8% (odds ratio 1.08, 95% CI 1.06 to 1.10). Being overweight before pregnancy carried about three times the odds (3.24, 1.93 to 5.46) and obesity before pregnancy about four times (3.75, 2.22 to 6.34). More than half of the women, 416 of 774, gained more than the national recommendation during pregnancy.
Breastfeeding to six months was associated with lower odds of holding 20 lb or more, and women who retained less had breastfed for longer on average (15.2 weeks against 8.9 weeks). That is an association in one cohort. It is not a reason to breastfeed and not a reason to feel you failed if you did not; this stage covers the gap between breastfeeding intention and what happens separately.
The study also reports that moderate exercise was associated with less retention. It prints an odds ratio for that finding whose value falls outside its own confidence interval, which means one of the two numbers is a typographical error that cannot be repaired from the paper. So this guide gives you the direction and no number. That same paper heads one table with 744 participants and uses 774 everywhere else; 774 is the number the analysis actually used.
A very different population, for comparison. The Danish National Birth Cohort followed 47,966 women who gave birth between 1997 and 2002, of whom 73% had a pre-pregnancy BMI under 25. At eighteen months after birth, compared with before pregnancy:
| Change by 18 months, in BMI units | Women | Share (calculated by this guide from the published counts) |
|---|---|---|
| Lost more than 1 unit | 9,948 | 20.7% |
| Within 1 unit either way | 27,178 | 56.7% |
| Gained 1 to 2 units | 6,823 | 14.2% |
| Gained more than 2 units | 4,017 | 8.4% |
These two pictures are not in conflict and they are not interchangeable. In a lower-weight European population, most women were back within a BMI unit at eighteen months. In a low-income American population with a higher starting weight, three quarters were still heavier at twelve months. Neither is a national figure for the other country, and neither is a statement about you.
Why weight is followed at all. The Danish study also tracked health for up to sixteen years. Compared with the women who stayed within a BMI unit, hypertension ran at 16.8 cases per 10,000 woman-years in the stable group, 21.0 in those who gained 1 to 2 units, and 22.1 in those who gained more than 2. Those are the absolute rates, printed alongside the relative ones, which is rare and worth using: a 31% relative increase here means moving from about 17 to about 22 cases per 10,000 woman-years.
"Can we talk about my blood pressure and my numbers rather than a target weight?"
"I would rather not be weighed today. Is there a medical reason we need to?"