SEOUL—When South Korea amended its adoption law in August of last year, it was intended to reduce unregistered adoptions of children overseas.

A year later, it appears to have accomplished that with an unintended side effect: a drastic increase in the number of babies abandoned anonymously by their mothers.

In the first seven months of this year, 152 infants were abandoned in South Korea, up from 62 in the same period of 2012, according to Ministry of Health and Welfare data. The increase has been attributed by some to the new Special Adoption Law, which stipulates that infants can't be put up for adoption without their births being registered with the government. It also requires that mothers remain with their newborns for a minimum of seven days before putting them up for adoption.

Activists for adoptee rights and government officials argue that more transparency was needed in the adoption process and question the causal link between the new law and the rise in abandoned infants.

Large numbers of babies were sent overseas from South Korea for adoption in the wake of the Korean War of the early 1950s, when the country was poor and rebuilding after the disastrous conflict. After the war, the country underwent rapid development but continued to send thousands of children a year abroad for international adoptions through the 2000s.