Video shared by the hospital shows the 7-ounce embryo had developed small feet, but few other features.
Doctors said the boy is recovering and is expected to be released from the hospital soon.
This is not the first instance of a case like this as a 26-year-old woman who underwent surgery in Los Angeles earlier this year for a suspected brain tumor was told after waking that the removed growth was a teratoma, or embryonic twin.
Yamini Karanam joked the tumor, which was not cancerous, was an "evil twin sister who's been torturing me for the past 26 years."
A medical team at Queen Elizabeth Hospital in Hong Kong released a study early this year detailing the case of an infant girl born "pregnant" with twin siblings in a condition known as "fetus-in-fetu."
"It was almost impossible to detect during the prenatal checkup, as the embryo inside the baby was too small," Dr. Yu Kai-man wrote in the Hong Kong Medical Journal. "Since it is impossible for the little girl to have conceived the pregnancy on her own, the fertilization of the twin fetuses, of course, belongs to her parents, which has gone to the wrong place."