It only required a glance at the ultrasound for the doctor to know that he was looking at identical twins.

The positioning of two amniotic sacs attached to one placenta was the giveaway. It would be a couple of months before he could tell the mother whether to expect two boys or two girls.

What seemed certain was that her babies would share a biological sex. And yet at 14 weeks when Dr. Nicholas M. Fisk, a maternal fetal medicine specialist at the Royal Brisbane and Women’s Hospital at the time, i nspected the ultrasound, he saw something perplexing.

One boy. One girl.

Sharing a single placenta.

“It doesn’t add up,” Dr. Fisk recalled thinking.

As it turned out, the twins were neither fraternal nor identical. They fell into a third rare category known as semi-identical or sesquizygotic twins. Although it would take several years to prove, he was looking at the first set of semi-identical twins to be identified during pregnancy, according to a paper published last week in the New England Journal of Medicine.