Some experts think it's because a woman is less likely to ovulate while she's breast-feeding.
There were years, he says, when there was literally nothing said about breast-feeding at all.
"It's kind of crazy that only 20% of people reach the recommended duration of breast-feeding, " says Flaherman.
The group's co-founder, Trysta Chavez, said the photos were meant to promote World Breast-Feeding Week in August.
BBC: US military in row over Facebook breast-feeding picture
Testosterone goes down for some women after the delivery, and breast-feeding releases the hormone prolactin, which inhibits arousal.
The Washington National Guard has criticised two servicewomen for being photographed in military uniform while breast-feeding their babies.
BBC: US military in row over Facebook breast-feeding picture
National Guard spokesman Captain Keith Kosik told the BBC that there was no regulation against breast-feeding in uniform.
BBC: US military in row over Facebook breast-feeding picture
"When most of the people who are currently leaders were in training, breast-feeding was really uncommon, " says Stuebe.
Promoting breast-feeding, for example, costs very little and is proven to save lives.
La Leche League International, a breast-feeding advocacy organization, said losing the nursing mothers' provision would set back its cause.
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In Scandinavia (where else?), there are many more women legislators, and day nurseries and breast-feeding in public are routine.
For much of human history, a woman's reproductive life was dominated by pregnancy and breast-feeding, both of which block menstruation.
This made Dr. Geske Ursin and her colleagues wonder whether the hormonal changes associated with pregnancy and breast-feeding were somehow the reason.
Researchers thought that only hormone-responsive breast cancers would be affected by breast-feeding.
Breast-feeding can also make it easier for women to shed pregnancy pounds.
"It was clear that none of the doctors or nurses knew enough about breast-feeding to figure out what was happening, " Kelly says.
When women have trouble breast-feeding, they're either prodded to try harder by well-meaning lactation consultants or told to give up by doctors.
Of course, we're low on those trained providers, but there are more every day, as medical schools begin to adopt breast-feeding curricula.
Breast-feeding advice, food supplements and better hygiene all make a big difference.
Our mission is to help raise awareness, educate and support breast-feeding mothers.
BBC: US military in row over Facebook breast-feeding picture
HIV-positive mothers can still pass the virus on by breast-feeding, so they would need to be provided with formula and taught about bottle-feeding.
Whatever the reason, the bottom line is clear: Breast-feeding offers broad protection against both major types of breast cancer in women of all ages.
The Healthy People 2000 initiative introduced in the US has set a target of 75% of American infants breast-feeding for at least six months.
They've successfully lobbied to include breast-feeding issues on the exams for the American Board of Obstetrics and Gynecology and the American Academy of Pediatrics.
Tanya Lieberman, a lactation consultant who writes about scientific research for breast-feeding advocacy organization Best for Babes, says she's "a little confused" by the results.
Not breast-feeding is, nevertheless, an effective addition to pre-natal antiviral drugs.
And at three months, 79% of the formula group was exclusively breast-feeding, significantly more than the 42% of moms in the group originally instructed to breast-feed.
Breast-feeding can certainly help provide immune protection against multiple illnesses, but not if a mother does not have immunity to a certain disease such as pertussis.
"We had looked at giving birth early, giving birth late, as well as breast-feeding, " says Ursin, who's on the faculty at the University of Southern California.
The chance an infected pregnant woman will transmit the virus to her baby during gestation, birth or breast-feeding ranges from 15% to 45%, according to WHO.
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