Mrs Williams is believed to be the youngest mother of conjoined twins in Britain.
Travelling to a new country with friends on holiday is also not as straightforward for conjoined twins.
Conjoined twins are rare and take place at the rate of about one in 400, 000 live births.
Any operation to separate conjoined twins is a highly complex and dangerous process.
The overall survival rate of conjoined twins is somewhere between 5% and 25%.
He pioneered surgery on newborns and successfully separated three sets of conjoined twins.
Conjoined twins are created just a few days after they are conceived - most likely by the incomplete splitting of the fertilised egg.
Conjoined twins are very rare - it is thought one in every 200, 000 births - and around 40-60% of these births are delivered stillborn.
Great Ormond Street, where the surgery on Faith and Hope was carried out, is the most experienced centre in Europe for separating conjoined twins.
That is because B-mesons are sometimes born as quantum-mechanically conjoined twins.
Before becoming the nation's chief doctor, Dr. Koop was famed for separating conjoined twins at Children's Hospital of Philadelphia, where he was surgeon-in-chief for three decades.
Historical records over the past 500 years detail about 600 surviving sets of conjoined twins with more than 70% of those surviving pairs resulting in female twins.
The ProLife Alliance urged British doctors to follow the example of their counterparts in Italy, who are also dealing with a pair of conjoined twins who share a heart.
Conjoined twins are very rare - only one in every 2.5 million births - and only 5% of conjoined twins are craniopagus, which means they are fused at the head.
BBC: Conjoined twins separated by Great Ormond Street doctors
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