Actress Olivia Munn revealed on a podcast in 2016 that she had frozen her eggs. She was 35 at the time. Picture: AP

Washington - Actress Olivia Munn revealed on a podcast in 2016 that she had frozen her eggs. She was 35 at the time. "Every girl should do it," she said confidently. "For one, you don't have to race the clock anymore. You don't have to worry about it, worry about your job or anything. It's there."

When some celebrities, fertility clinic representatives and other proponents talk about egg freezing, they often make it sound like a sure thing. You go in, get your eggs put on ice, then go back a few years later. A doctor fertilizes them, puts the embryos inside you, and months later a baby pops out.

If only it were that easy.

When it comes to human reproduction, Mother Nature is wildly inefficient. On average, a woman is born with about 2 million eggs - but only some of them will have any chance of becoming children. Likewise, when you freeze a batch of eggs, that's no guarantee any or all of them will make it through the next steps. One egg frozen does not necessarily equal one child.

That simple truth leads to a complicated question: Just how many eggs should you freeze to have a decent chance of having a baby in the future?

At many clinics, women used to be advised to aim to freeze 10 to 20 eggs, no matter their age or situation. But that never made much sense given that eggs decline in quality and quantity as a woman ages.

As the industry matures and data builds about successes and failures, some providers are recommending a more individualized approach, including assessing what a woman is able and willing to spend. For some women, a 50 percent chance may be worth it. For others, 50 percent is not enough.

"The devil is in the details," said Joshua Klein, chief medical officer at Extend Fertility, a New York City-based clinic that specializes in egg freezing. "It always bothered me, and still bothers me, that women who come to me about a second opinion do not have clarity on how many eggs is a reasonable target."

"Ten eggs for a 30-year-old is a very different thing than 10 eggs for a 40-year-old," he explained.

Janis Heidi Fox, an obstetrician-gynecologist at Brigham and Women's Hospital in Boston, and Randi Goldman, a clinical fellow at Harvard Medical School, are among the first to try to assign some probabilities to this process based on a woman's age and the numbers of eggs retrieved."

According to their analysis, a woman who is 35 with 10 eggs has a 69 percent chance of a baby. At age 37, she has a 50-50 chance. And at age 39, she has a mere 39 percent chance.

Using frozen eggs is a multistep process, with many eggs or embryos lost along the way. First the eggs have to be thawed, then they are fertilized with sperm. Then they are left in a petri dish to grow into what are called blastocysts. A proportion of those blastocysts can be abnormal. Some of the embryos have genetic abnormalities. Then the apparently healthy embryos are transferred into a woman's body. Some attach to the wall of the uterus, and others don't. If they do, miscarriage is still a risk.

Fox and Goldman estimated the percentages of eggs and embryos that make it through the various procedures and screenings involved based on their knowledge of the previous research. Each of those percentages represents a fraction of the eggs or embryos that have survived the previous step.

Thawing: 85-95 percent

Fertilization: 70 percent

Blastocysts: 40-50 percent

Normal chromosomes: 57 percent for eggs harvested when the woman was age 35 and under, 13 percent for 44 and over

Implantation: 70-75 percent

Live birth: 60 percent (assuming chromosomal testing and the other steps above go smoothly)

Some fertility specialists refer to this as the inverted pyramid of IVF, and they say women are often surprised when they finally understand how it works.

"The whole power of this technology is in numbers," Goldman said. "The more eggs you have, the better our chances of selecting the better eggs and the better the chance of having a baby."

The Washington Post