"I feel proudest, because they had lost hope for life", He said, when challenged by several peers at the conference (live stream below).
But details of the experiment, which has not been independently verified, triggered an immediate backlash and He said the trial had been halted. China's National Health Commission said He's activities would be investigated and any wrongdoing "resolutely dealt with", according to Xinhua.
Jennifer Doudna, a University of California-Berkeley scientist and one of the inventors of the CRISPR gene editing tool which Mr He claimed to have used, said: "This is a truly unacceptable development".
He gave a partial apology in front of a packed auditorium at the Second International Summit on Human Genome Editing in Hong Kong, although the contrition seemed to be more for the information about the births coming out before his research had been vetted by the scientific community, rather than for having carried it out.
He, who earlier sparked worldwide debate after revealing his unprecedented trial, defended his research at the summit on Wednesday and revealed there was another potential pregnancy of a gene-edited embryo. No doubt, China has been at the forefront of gene-editing research for a few years now.
He explained that eight couples - comprised of HIV-positive fathers and HIV-negative mothers - had signed up voluntarily for the experiment; one couple later dropped out.
The Southern University of Science and Technology is investigating the project, as are local officials in Shenzhen.
Prof He's university said it was unaware of his experiment.
In a statement to the Hong Kong stock exchange on Tuesday, the group said preliminary investigations indicated the signatures on the application form circulated on the internet are "suspected to have been forged, and no relevant meeting of the Medical Ethics Committee of the hospital in fact took place".
Nobel Prize laureate David Baltimore, who also attended the genetics conference, criticised the secrecy of He's work. Many attendees said there were other ways to prevent the spread of HIV. Although his appearance had been previously scheduled, Lovell-Badge said He had earlier "sent me the slides he was going to show in this presentation and it didn't include anything that he is going to talk about today".
The Stanford-educated researcher said the twins' DNA was modified using CRISPR, a technique which allows scientists to remove and replace a strand with pinpoint precision.
Harvard Medical School dean Dr. George Daley says it would be unfortunate if a misstep with a first case led scientists and regulators to reject the good that could come from altering DNA to treat or prevent diseases.
Food and Drug Administration Commissioner Scott Gottlieb also weighed in, emphasizing in a Twitter post the need for "more than just laws" to ensure CRISPR-Cas9 and other gene-editing technologies aren't misused or abused.