BEIJING — China is vowing to strengthen enforcement to prevent sex-selective abortions and close a yawning gender gap in a country that already has tens of millions more boys than girls.

The pledge is in the outline of a plan for childhood development through 2020, but it has no specifics. The plan said authorities would increase efforts against the non-medical use of ultrasound tests and abortion of fetuses based on gender.

Spurred by the one-child policy and a traditional preference for boys, sex-selective abortion has created a male-female ratio at birth in China of about 119 males to 100 females, with the gap as high as 130 males for every 100 females in some provinces. In industrialized countries, the ratio is 107 to 100.