LOI, Vietnam — They had no plan to break barriers or cause trouble. But 30 years ago in this bucolic village in northern Vietnam, the fierce determination of one group of women to become mothers upended centuries-old gender rules and may have helped open the door for a nation to redefine parenthood.

One recent morning in Loi, as farmers in conical straw hats waded quietly through rice paddies, a small group of women played with their grandchildren near a stream. Their husbands were nowhere to be found, not because they perished in the war, but because the women decided to have children without husbands.

The women’s story began during the American War, as it is called here, when many put the revolution before their families. As peace settled more than a decade later, it became clear that they — like so many of their generation — had sacrificed their marriageable years to the war.

At that time Vietnamese women traditionally married around 16, and those still single at 20 would often be considered “qua lua,” or “past the marriageable age.” When single men who survived the war returned home, they often preferred younger brides, exacerbating the effects of a sex ratio already skewed by male mortality in the war. According to the Vietnam Population and Housing Census of 2009, after reunification in 1979 there were on average only about 88 men for every 100 women between 20 and 44.