A set of conjoined twins have been separated in a US hospital after flying from Nigeria for the life-saving procedure. The one-year-old girls, Miracle and Testimony Ayeni, were conjoined at the lower half of the body.  They underwent an 18-hour operation at Le Bonheur Children’s Hospital in Memphis, Tennessee.

The hospital announced the landmark procedure on Wednesday morning, with more information to follow later today.

It comes just weeks after twins from Illinois conjoined at the head were separated at New York City’s Montefiore Hospital. The incredibly delicate operation to separate Jadon and Anias McDonald took 27 hours, and the lead surgeon has admitted he considered stopping halfway through.  However, both boys are now recovering and showing strong signs of development.

Conjoined twins are formed in a similar way to identical twins. In the case of identical twins, the mother releases one egg that fertilizes and splits.  They have their own amniotic sacs, and they are genetically identical.  With conjoined twins, the same process occurs.  However, the separation process after fertilization does not complete.  The embryo starts to split to form identical twins but instead forms one fetus.  Since only one egg is involved, conjoined twins are always the same gender.  Conversely, with fraternal (non-identical) twins, the mother releases two eggs instead of one, both are fertilized.  She then carries two babies, each in their own amniotic sac. They are not genetically identical.