One reason why determining survivability is so difficult is because of arbitrary rules. For example, the US and UK guidelines define one of the indicators of miscarriage as an empty gestational sac of various sizes (16 mm in the US and 20 mm in the UK). “This leaves 4 millimeters of room, a large margin given the size of the embryo. An American radiologist would diagnose a miscarriage, but a British radiologist would not,” Clancy wrote.
Doctors usually determine early survival based on gestational age. Gestational age is usually calculated from the first day of a pregnant person’s last menstrual period. (According to this method, the fetus is “born” several weeks before the sex or procedure that will carry it.) This measurement, known as the “last menstrual period” method, is highly inaccurate because it assumes a standard 28-day cycle. Women with cycles longer or shorter than four weeks routinely produce embryos that appear “too small” or “too big.” If the timing is off by a week or two, even a healthy early pregnancy can seem like things aren’t going as planned.
One of the patients interviewed by Clancy was given two estimated fetal ages at different times, and the estimated fetal age was 4 weeks. Doctors estimated that the patient was about 10 weeks pregnant from her last menstrual period, but judging by the size of the embryo, she was about 6 weeks pregnant. When the patient started bleeding, which is normal in early pregnancy, doctors didn’t know whether this was a six-week pregnancy with a chance of survival or a ten-week pregnancy with a miscarriage. (After two weeks of confusion, she was diagnosed with a miscarriage and continued to bleed for another six weeks.)
Four weeks has a very large margin of error. A person can be diagnosed with an ongoing pregnancy or a miscarriage, and the diagnosis depends on where the person lives, which hospital they go to, and who their doctor is. However, many pregnancies exist in what Clancy calls the “critical intermediate area” between viable and non-viable, which clinicians seek to capture with the term intrauterine pregnancy of unknown viability, or IPUV. The term can be applied to pregnancies without a yolk sac, a yolk sac but no visible embryo, or an embryo but no detectable heartbeat, which could be diagnosed as a miscarriage. Clancy cites one study that found that out of about 1,000 IPUV cases collected in the UK, about a fifth were found to be fertile.
IPUV categories are useful because “unviable” often provides a more honest description of the early stages of pregnancy than either viable or nonviable. But the uncertainty that comes with an IPUV diagnosis compounds the limbo that often accompanies the experience of miscarriage, during which most people “have no idea whether to be hopeful or start grieving,” Clancy writes.
This waiting is not just part of miscarriage. Every pregnancy involves an important wait: waiting for a heartbeat. Wait for the embryo to pass through multiple stages from successful implantation to approval of the placenta. Wait for various test results after 8, 12, and 16 weeks. Or do nothing special and just hope for the best.
This is one of the reasons why online pregnancy announcements are so weird. In other words, it suggests that the experience of “finding out that you are pregnant” is a special moment. A lot can happen between peeing on a stick and the baby being born. Confirmation of pregnancy is less a “discovery” experience than a process of eliminating all other possibilities and gradually working toward the most likely outcome. “Life is a movie, not a photograph,” are the words of Clancy’s own doctor.
Many early miscarriages occur as part of the natural process of discarding a “mistake.” In some cases, our bodies define these errors without our knowledge. In a chemical pregnancy, a person is pregnant, i.e. gamete fusion and embryo implantation have been successfully completed, but miscarriage occurs at approximately the same time as her period is due. If I hadn’t been looking, I never would have known I was pregnant.