And he insisted that the parents of the twins and seven other couples who had participated in his research were fully informed of the risks involved, and that they understood what was being done to their embryos.

“The parents were informed of the implication of this,” Dr. He said. “We reminded them of the option to leave the trial without implantation, or to choose the embryos. The couple elected to implant these embryos to start a two-embryo pregnancy.”

Showing a series of slides, he quickly described what he said was three years of work involving mice, monkeys and then human embryos. He used the editing technique, Crispr-Cas9, to disable a gene, called CCR₅, which creates a protein that makes it possible for H.I.V., the virus that causes AIDS, to infect people’s cells.

Dr. He said that with the help of an H.I.V./AIDS organization in China, he recruited eight couples in which the man had H.I.V. and the woman did not. One couple dropped out and another achieved a “chemical pregnancy,” a pregnancy which fails soon after the embryo is implanted in the womb.

After editing the CCR₅ genes, he said he used in vitro fertilization to create embryos that were resistant to H.I.V. The goal, he said, was to engineer babies who would not be vulnerable to H.I.V. infection.

Many scientists have noted that there are other, simpler ways to protect newborn babies of an infected parent, especially an infected father, from getting H.I.V. Embryo editing should be used only to prevent or treat dire medical conditions that cannot be addressed in any other way, they said.

But Dr. He suggested that the parents of the twins, especially the H.I.V.-positive father, saw the procedure as a way to recapture their reason for living.