It’s estimated that almost half of all pregnancies end in miscarriage — many occurring just weeks after conception. Once a woman knows she’s pregnant, the chance of a miscarriage is 15-20 percent, with the majority happening in the first 10 weeks.
That’s a shockingly bad track record for a process as vital and age-old as baby-making. The question of why early pregnancy is so perilous has intrigued scientists and doctors for a long time. Recently, though, the answers have become clearer.
Miscarriages can happen for many reasons, like a hormonal problem affecting the mix of chemicals in mom’s body, a physical issue with her reproductive system, or an infection early in the embryo’s development. Obesity and heavy caffeine intake have also been linked to lost pregnancies.
But the culprit in at least half of miscarriages is something beyond a mom’s control and often completely unrelated to her health or lifestyle. Chromosomal abnormalities – the addition to or subtraction from the normal set of 46 chromosomes, or tightly-pack bundles of DNA in a fetus’ cells – are responsible for the bulk of halted pregnancies. In fact, some research has estimated that faulty chromosomes make for up to 80 percent of spontaneous miscarriages in the early weeks if a mom is over 35.
When I talk to friends who have had miscarriages, the question of why and what went wrong seems to linger with them — understandably so. But the comforting fact (or the disconcerting one, depending on how you look at it) is that most of the time, chromosome mishaps are entirely random. They usually have nothing to do with mom or dad’s genes, and don’t much affect a couple’s chance of having a perfectly healthy pregnancy in the future.
Still, humans have been creating life from egg and sperm for millennia, presumably perfecting the process as they go. So, in our modern world, why would so many embryos fall victim to fatal chromosomal glitches?
Earlier this year, scientists got a peek at the answer to this by studying conception in mice (whose baby-making process is similar to ours) and watching what happens before sperm finds egg. In a Current Biology article, the researchers reported that the process of making an egg cell is usually where the problem lies. Normally, when egg cells divide and get ready to meet their other half, they create two side-by-side line ups of 23 chromosomes (46 in total) — a two-lane genetic highway in the middle of the cell. When everything is in place, a chemical signal tells the cell it’s time to divide and each daughter cell ends up with its 23. Later, when a sperm comes into the picture with its own 23, a genetic set is complete.
But often the signal, called the “spindle assembly checkpoint,” jumps the gun and tells the egg cell to divide before everything has lined up. That means one daughter cell might have 24 chromosomes and one 22 — either way, if a sperm finds one of these eggs, the numbers are off. The fertilized eggs start to divide and grow, but can’t make it very far, and a miscarriage results. The exception is when the off-count happens to be an extra chromosome 21, in which case the fetus can flourish, but the baby has Down syndrome. As a woman gets older, the signal seems to get more trigger-happy (which is why the risk of chromosomal problems goes up with age), but the misstep can happen to a woman in her 20s, too.
Scientists wonder if evolution has allowed error-prone eggs to slip by because they’re so precious: better to let them divide and try their luck than to keep them tied up and unavailable. Sperm, on the other hand, seem to have a more air-tight quality control system.
Of course, knowing the biology involved doesn’t make it any easier for a couple who loses a pregnancy. But it helps explain why miscarriages are so common and in many cases unrelated to anything a mom does or doesn’t do in the early weeks. It’s true that to stack the odds of a healthy pregnancy in our favor, women are recommended to be healthy themselves and start taking vitamins before a baby is made. But in the end, for a lot of pregnancies, there’s a delicate and arbitrary force of nature involved that can lead things astray, and for the most part, that part of the process is completely out of our hands.