Ryleigh Shepherd was conceived in 1998, the same year as her 11-year-old twin sisters, but she wasn't born until 2010.
The three girls from Walsall, in Great Britain, who were born more than a decade apart in two different centuries, are actually fraternal triplets born through in vitro fertilization (IVF).
Ryleigh came from the same batch of embryos that had allowed her parents -- Lisa and Adrian Shepherd -- to give birth to twins Megan and Bethany.
British experts say they know of no other case in their country in which three siblings from the same round of fertility treatment have been born with such an age gap.
The longest interval between freezing and conception was in the case of a woman from New York City whose embryo had been stored for 20 years, according to a report in the journal Fertility and Sterility.
"It seemed strange to think that we were using embryos that we had stored all those years ago, that were conceived at the same time as the girls," Lisa Shepherd, 37, told Britain's Daily Mail newspaper.
"We knew that if we had another baby it would in effect be the girls' triplet as they were all conceived at the same time," she said.
The girls look exactly alike, according to their mother. "It was uncanny."
How long embryos can be frozen and still viable is still not known, but American fertility experts say they have great confidence in the success of new reproductive techniques.
"It's incredibly common for people to go back and second and third time," said Barbara Collura, executive director of RESOLVE, the National Infertility Association. "There have been recorded cases of kids born far longer apart. This doesn't tip the scales."
Fertility experts estimate that about 400,000 embryos are currently in frozen storage in the U.S., and a more comprehensive survey will be underway in the spring.
Couples undergoing IVF must make a decision about saving the eggs at a cost of about $200 to $300 a month for future use. Many couples choose to donate their frozen embryos to other infertile couples, according to Collura.
"When they are done with their family building and years pass, they can donate to another couple," she said.
Cryogenic techniques have improved significantly since the 1990s, when the Shepherd twins were conceived. Experts speculate that frozen embryos can last "15 to 20 years or more," said Collura.
Women in their early 30s who are undergoing IVF can be hopeful that their frozen embryos might be used later on.
Such was the case with the Shepherds, who were married in 1994 and wanted to start a family. But Lisa Shepherd had been diagnosed with endometriosis and polycystic ovaries, and doctors told them her chances of getting pregnant were not promising.
"I was given drug treatments to help me conceive, but nothing worked," she said. "It was devastating."
In September 1998, the couple underwent fertility treatment; 24 eggs were collected and 14 were successfully fertilized with Adrian Shepherd's sperm.
Couple Waited a Decade to Use Frozen Embryos
Two embryos were implanted and the remaining 12 were placed in frozen storage. The couple said they didn't "get their hopes up."
Soon the couple was expecting twin girls, who were born by Caesarian section.
When the twins were 9, the family began to think about having another baby.
"We had been so busy raising the twins that it wasn't until then that we stopped to think about having another one," said Lisa Shepherd, whose husband, 45, is an engineer.
"'So we asked the girls what they thought about having another addition to the family and they really wanted it."
In December 2009, the couple returned to Midland Fertility Clinic for another cycle of IVF, using the 10-year-old embryos. Ryleigh arrived in November.
"The girls are thrilled to have a sister -- and they know that she was conceived at the same time that they were, but has been in the freezer," said Lisa Shepherd.
Doctors say there are few safety concerns about using long-frozen embryos.
"Once an embryo is frozen, it's essentially frozen in time," said Dr. Jani Jenson, a reproductive endocrinologist at the Mayo Clinic. "As a group, children born with IVF are one of the most scrutinized cohorts. "
Because these technologies are only 40 years old, no one knows how these babies will age, but all other studies have indicated IVF is safe.
"The data we know from fresh and frozen transfers are that it doesn't put them at any unique or known risks like learning disabilities or birth defects," said Jensen.
The Shepherd case is, "interesting, and yet mundane," said Dr. Ellen Clayton, director of the Center for Biomedical Ethics and Society at Vanderbilt University. "They are just siblings and this is clearly a family where everyone resembles everyone else. The twins look like two peas out of a pod and this is the third pea."
Ethical concerns have been raised about risks involving IVF, according to Clayton, but the "relevant metric" is how parents make other choices about having children.
"The current thinking is that parents are free to make pretty broad choices," she said. "They are free to continue a pregnancy with Down syndrome. People make all kinds of choices that are potentially risky."
As for the Shepherds, said Clayton, "God bless them. This looks like a happy family that is now even happier."