Soon I found an eye-opening study that all but mocked the 3 percent odds we were given of carrying a baby to term.
Researchers at St. Mary's Hospital in London, which runs the world's largest miscarriage clinic, closely followed 201 women, each of whom had had at least three losses and had become pregnant again. The doctors had not discovered any underlying causes for their losses. Nearly 70 percent of these women carried their pregnancies to term without any interventions, save for the expert care they received at this clinic.
Transcript: Dr. John Larsen, a specialist at George Washington University Medical Center, answered questions about high-risk pregnancies.
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I had never heard of miscarriage clinics when we were in the midst of our miscarriage woes. Consequently, I did not know that some of these clinics strongly encourage their patients to have their miscarriages analyzed for chromosomal abnormalities. Such information could have provided us with a much better understanding of our options -- and our odds of eventual success -- as well as some peace of mind about the wildly confusing and emotionally draining labyrinth that you and your partner enter when your bodies fail at reproduction.
About half of all pregnancy losses are due to chromosomal abnormalities, and these losses, like Shannon's, typically occur within the first trimester. But chromosomal abnormalities in miscarried pregnancies, it turns out, often are actually good news for couples. If a woman miscarries an embryo that has too many or too few chromosomes, then everything typically is working right: She is becoming pregnant, but losing a baby that has virtually no chance of surviving.
Why weren't we offered this so-called "karyotype" test to determine if Shannon's miscarriages were due to chromosomal mayhem? I have no idea. It can't be monetary concerns alone. The test costs less than $600, and several studies have shown that it benefits health plans to offer a karyotype, because women who have chromosomally abnormal losses need not be encouraged -- as Shannon was -- to have expensive (and invasive) additional tests that attempt to detect problems.
Under the Microscope
I soon became fascinated by the fundamental biology driving the chromosomal blunders involved with so many miscarriages. Why are they so common, and why do they increase as a woman ages? I found an elegant, startling answer.
A girl makes all the eggs that she will carry for life while she is still a fetus. Those eggs carry 46 chromosomes. A human has 46 chromosomes, so the egg must throw away half its genetic cargo when it meets a sperm, which shows up to the fallopian boudoir with the requisite 23 chromosomes of its own. But the egg tosses those extra chromosomes overboard only at conception.
So when an egg meets a sperm, the egg is decades old. As an egg ages, the mechanism that separates chromosomes becomes wobbly and inefficient, sometimes mistakenly tossing out, say, 22 or 24 chromosomes instead of 23. Both of those scenarios lead to embryos that almost always miscarry. (Some babies do survive with abnormal chromosomes -- Down syndrome is caused by three copies of chromosome 21, for example -- but the vast majority cannot survive.) It further raised my eyebrows to learn that, technically, it's not age driving miscarriage but the woman's proximity to menopause.
The female fetus makes about 7 million eggs and they begin to self-destruct even before birth. A female will ovulate only about 400 to 500 eggs during her reproductive years, so the system has a huge oversupply built in. But the pool of eggs a woman has in her ovaries secretes chemicals that promote the health of the few eggs that do ovulate. As the pool of eggs shrinks and the critical chemical secretions decrease, the health of the ovulated eggs suffers.
Once only about 1,000 eggs remain, a woman enters menopause. Given that women go through menopause at anywhere from 40 to 60 years old, each woman thus has her own unique biological clock. Ultimately, then, each woman has her own miscarriage clock: The closer her ovaries come to having 1,000 eggs in the pool, the more likely she will have a chromosomally abnormal embryo and a loss.