In the United States, narcotics offenses are a leading cause of female imprisonment, he said.

Ms. Wang estimated China’s female prison population at 4 to 5 percent of the total, but she cautioned that she did not have exact figures, with so much about China’s prison system a secret.

And Mr. Kamm also noted that the new clauses did not apply to women in China’s large system of re-education through labor, or “laojiao,” an extrajudicial form of detention that snares many thousands every year.

“I don’t see anything in this law that helps women in laojiao,” he said.

Facing rising numbers, pressure is growing to better manage women’s needs in prison, advocates say. They point to the “Bangkok Rules,” adopted by the United Nations in 2010, as an important step.

Formally known as the United Nations Rules for the Treatment of Women Prisoners and Non-custodial Measures for Women Offenders, the agreement calls for a range of improvements, including recording new inmates’ experiences of sexual abuse before entering prison and providing more water for washing, especially for menstruating women.

Ms. Wang says China already does the latter. In fact, she said, in China’s paternalistic system, jailed women have better conditions than men. “The government has a way of protecting women that’s traditional,” she said.

“For example they have more hot water to wash themselves, their lower bodies. There are fewer people in each room, about four to six or six to eight, while the men are, say, six to eight or 10,” she said. “They get more space to keep personal possessions,” she added.

When the police arrest a woman who is pregnant, “she cannot be sent to prison. She doesn’t go into detention either,” said Ms. Wang. “Generally she stays at home and is monitored in her district. Depending on the severity of her crime, she is either monitored by the police or by other local officials.”