Healthy twin girls -- Elizabeth and McKinney Caudill -- are the first babies in Minnesota to be born from eggs that were frozen and then thawed before being fertilized in a petri dish.

Doctors at the Mayo Clinic in Rochester who treated the parents are now offering egg freezing as an option for those with a good reason to use it.

Egg freezing has growing appeal as a way to preserve fertility for women, or, as in the case of the twins' parents, to avoid the wrenching decision of what to do with excess frozen embryos that will never become children. While frozen embryos remain by far the most common and successful choice for use in vitro fertilization (IVF), egg-freezing technology is rapidly improving.

Mayo's is just one of many IVF clinics nationally trying to improve success rates for frozen eggs to cater to what many experts say is an enormous market. So far, only 200 to 300 babies have been born from frozen eggs worldwide. But with millions of women delaying child bearing and egg freezing so far the only way to bring a woman's biological clock to a halt, experts say the technology could revolutionize women's reproductive lives as much as the birth control pill did 40 years ago -- for those who can afford it.

"For women who are sure they are going to go through menopause from cancer treatments, or for women in their mid-30s who don't see a partner on the horizon, there really aren't other options," said Dr. Elizabeth Ginsberg, president-elect of the Society for Assisted Reproductive Technology (SART) and a fertility doctor in Boston.

The embryo dilemma

The twins' parents, Ceresa and Jonathan Caudill, were reluctant to freeze embryos if those not used in IVF might be destroyed. It's a concern Dr. Charles Coddington, head of Mayo's fertility clinic, hears from many of his patients.

The Caudills started IVF in the fall of 2006, a few years after the birth of their son, Jonathan Jr., now 5. In IVF, the woman's ovaries are stimulated with powerful fertility drugs to produce a dozen or more eggs in one cycle. The eggs are surgically removed and then fertilized by sperm in a petri dish. Doctors implant one or more of the resulting embryos in her uterus and freeze others to be used at a later time.

But the Caudills chose not to create more embryos than they could use at one time. "What we were thinking is that they are the smallest members of our family," said Jonathan Caudill, 36. Coddington asked if he could use the unused eggs for the clinic's ongoing research into egg freezing.

After two rounds of IVF and a miscarriage, the Caudills were reluctant to go through another series of painful and expensive fertility treatments.

"Dr. Coddington said, 'I think we have some eggs to work with,'" said Ceresa Caudill. "We had five."

How to freeze an egg

Unlike embryos and sperm, eggs do not freeze well. The body's largest cell, they are mainly water. When frozen, the water crystalizes and destroys the egg.

The Mayo Clinic uses a technique that involves first removing much of the water from the egg and then slowly freezing it. When it's time to thaw, the egg is warmed slowly so water can be re-absorbed.

About half survive the thaw, Coddington said. But only 10 to 15 percent survive the IVF process and eventually result in a live birth. Embryos, in contrast, survive the thawing process 90 percent of the time at Mayo's labs and result in a live birth about half the time.

In the Caudills' case, four of the five eggs survived the thaw. Three were fertilized and transferred to Ceresa Caudill's uterus. Two grew, and Elizabeth and McKinney were born in early June.

"They were very honest with us," Ceresa Caudill said. "I didn't think it would be successful."

Coddington said that one reason it was successful is that Ceresa, at 32, is relatively young. Younger eggs are more likely to survive the process.

Available for a price

The Mayo Clinic will now provide the service to anyone for whom it is appropriate, including women who simply want to delay child bearing, Coddington said. Egg freezing costs about $1,000, twice as much as freezing embryos.

Though Mayo is the first fertility clinic in the state to announce a live birth, two others are experimenting with egg freezing as well: Reproductive Medicine and Infertility Associates (RMIA) in Woodbury and the Center for Reproductive Medicine in Minneapolis.

Last year, RMIA helped a 48-year-old woman become pregnant with a younger woman's donor egg that was frozen before it was fertilized. That pregnancy, however, ended in a miscarriage, a RMIA official said.

Coddington and others at Mayo are also working with researchers at Northwestern University on a new freezing technique designed for young women and children whose fertility is destroyed by cancer treatments and who cannot take the powerful fertility drugs. Dr. Jani Jensen, also a fertility expert at Mayo, said that the effort is a response to the high survival rates among young cancer patients.

For breast cancer alone, survivorship is 90 percent," she said. "There are 250,000 reproductive-age women survivors of breast cancer."

That research involves freezing immature eggs in ovarian tissue, rather than those matured through fertility drugs. But that is more difficult, Jensen said, because the different types of tissue freeze at different rates.

"It's like trying to freeze a whole plant instead of just the seeds," she said.

Josephine Marcotty • 612-673-7394