The findings, reported in the journal Obstetrics & Gynaecology, add to evidence that at least some obese women may be able to shed pounds during pregnancy without harming themselves or their baby.
But researchers still caution that women should not start a weight-loss diet during pregnancy, and should instead talk with their doctor about any healthy diet and lifestyle changes.
The general recommendations from the Institute of Medicine (IOM), an advisory panel to the US government, say that obese women should gain 11 to 20 pounds during pregnancy.
That's less than the 25 to 35 pounds recommended for normal-weight women. But some researchers have criticized the IOM guidelines for not considering different levels of obesity.
A few studies have suggested that women who are severely obese can gain fewer pregnancy pounds, or even lose weight, without harming their baby or their own health.
In the new study, Dr Marie Blomberg, of the University of Linkoping in Sweden, examined medical records for more than 46,000 obese pregnant women who gave birth between 1993 and 2008.
She separated the women into three categories of obesity: class 1 obesity, which means a body mass index (BMI) of between 30 and 35; class 2 (a BMI between 35 and 40); and class 3 (BMI of 40 or higher).
Blomberg found that of women in the latter two groups, those who gained less than the IOM recommends - or even lost weight - were less likely to deliver a large newborn or need a C-section than their counterparts who did gain the recommended amount.
Among women with class 2 obesity, for example, 17 % of those who lost weight had a C-section, versus 24 % of those who gained the IOM-recommended amount.
And babies born to very obese women who lost weight seemed to fare as well as infants whose mothers gained the recommended amount.
They were no more likely to go into foetal distress or have a low Apgar score - a measure of breathing, heart rate and other health indicators right after birth.
The findings suggest that it is "at least not harmful" for very obese pregnant women to gain less than the IOM recommends, or possibly lose pounds, Blomberg told Reuters Health by email.
However, exactly how much weight loss might be safe is unclear. Blomberg did not have information on the range of weight loss among women in this study. And she said she is not aware of other studies that have suggested a safe range.
There may also be a tradeoff when obese women lose weight during pregnancy: an increase in the risk of having a baby who is small for gestational age - smaller than the norm for a baby's sex and the week of pregnancy during which she was born.
Blomberg found that of the most obese women (a BMI of 40 or more), those who lost weight were twice as likely to have a baby who was small for gestational age.
However, that bumped up their rate to what is seen among pregnant women generally. Overall, 3.7 % had a small-for-gestational-age newborn - compared with a rate of 3.6 % among all births in Sweden.
The bottom line, according to Blomberg, is that some weight loss may be "reasonably safe" for women who are severely obese.
However, she cautioned women against starting any kind of weight-loss diet during pregnancy. Instead, she said, they should aim to eat a balanced, healthy diet and get regular moderate exercise, like a half-hour of walking each day.
That is the general advice. But it's of "great importance," Blomberg said, for obese pregnant women to talk with their own doctor about any diet and lifestyle changes they might need to make.(Reuters Health/May 2011)