On the ultrasound screen, I could see 11-week-old twins—two shadows, like reflected commas, suspended in the patient's uterus. One embryo moved its arm. The other, slightly smaller, squirmed and floated head up. When I pointed them out, Linda was both shocked and overjoyed. "I can't believe it!" she said. "I'm going to have twins!" She propped herself on her elbows to see, the table paper crinkling beneath her. Outside the exam room door, the waiting room was filled with other pregnant women waiting for their exams. But here was an event. Twins.
In the women's center where I work as a nurse practitioner, we see twin gestations several times a year. We don't often see identical twins, but the occurrence of fraternal twins, resulting from the fertilization of two eggs by two different sperm, is more common and varies according to race, genetic factors, maternal age, and the use of fertility drugs. Once, we had a real surprise—fraternal twins fathered by two different men. In that case, the patient's two eggs had been fertilized by different partners, a rare occurrence that results when separate acts of intercourse take place within a brief period of time.
Linda's twins appeared to be fraternal—we could see that the embryos were encased in separate membranes—and her predisposition to twins was probably age-related: Linda was 39. Although it seems logical that twins would occur less frequently as women age, the opposite is true. The more pregnancies a woman has carried and the older she is, the more likely she is to conceive twins. An increase in one of the reproductive hormones that helps eggs mature, FSH, is probably responsible for the increased chance of bearing twins.
I told Linda we would schedule another ultrasound at 18 weeks and that during the pregnancy we'd monitor the growth of the babies with monthly ultrasounds. She understood that a twin pregnancy was more vulnerable than a singleton. Intrauterine fetal demise, premature delivery, and discordant growth of the twins were all distinct possibilities. In addition, because of Linda's age, her pregnancy was at a higher risk for genetic abnormalities. She was nonetheless encouraged by the ultrasound; she carried the sonogram picture of her twins in her wallet and showed it to everyone.
When she was 13 weeks pregnant, Linda returned to the clinic for a routine check. The resident who examined her at that time located two strong fetal heartbeats. Two weeks later, when Linda was 15 weeks pregnant, I picked up her chart, happy to have the chance to see her again. She was animated, and we chatted about the Internet exploration she'd been doing. She had started e-mailing other women with twins, gone window-shopping for strollers, and subscribed to a magazine for families with twins. Loving the idea of having twins, she'd already begun thinking in pairs—a double stroller, two cribs, two bikes, and, eventually, two college educations.
First, I felt Linda's enlarged belly. Then, after slathering her abdomen with lubricant, I listened for two distinct fetal heart tones. I found one twin's heartbeat right away. It was strong and steady at 140 beats per minute, exactly in the normal range of 120 to 160; fetuses, like other small creatures, have rapid heart rates. Linda and I smiled. The sound was like the gallop of a faraway pony or the wing beats of a hummingbird. Normally, it would have taken me only a moment or two to locate the other twin's heartbeat, but the minutes dragged on. No matter how intently I listened or how carefully I maneuvered the Doppler stethoscope over Linda's belly, I could find only one fetal heartbeat.
Linda asked, "What's the matter?"
My own heart was pounding as I tried to reassure her: "It's possible that I'm just not able to track down the second fetus. They move around pretty quickly." I didn't remind her that the miscarriage rate is high in twin pregnancies, so much so that women often lose one twin before they even know they are expecting. I didn't say that in one study, twin pregnancies were identified in 30 women but only 14 resulted in the birth of two infants. Or that I'd read that when twins were seen on women's early ultrasounds, less than 50 percent of those women went on to deliver two babies.
Linda and I moved to the ultrasound room. When we looked at the screen, we could see that one twin was viable, moving and rolling, but the other was shrunken and still. I hugged Linda as she wept.
I assured her that this was not the result of anything she had done, nor was this necessarily a harbinger of disaster for the other fetus. I told her we would do what we usually do: wait. In the back room, the residents and I no longer spoke of Linda's two fetuses but referred to the one that had silently succumbed as the "conceptus" or the "demise."
The next ultrasound showed that the viable fetus was thriving. This one was female, her features identifiable and her sex clearly visible. The discrepancy was marked: in one sac, a baby girl; in the other sac, the conceptus, like a dark half-moon. I gave Linda all the details I could: how many weeks the other fetus had lived, how its minuscule body might eventually become compressed into the membranes, or how it might be delivered alongside the daughter.
Four weeks later, I expected to see an even more discordant picture as Linda's baby girl continued to grow, dwarfing the conceptus. But the ultrasound revealed only the healthy child. The dead fetus, the failed twin, the forever-unknowable child, had vanished, leaving behind, like a cast-off jacket, its empty sac.
"Where did it go?" Linda asked, feeling her loss anew.
While the very early miscarriage or arrested growth of one developing ovum is common, what happened to Linda—the total disappearance of a twin later in pregnancy—is unusual. I gave her the simplistic explanation given in medical texts: "It was reabsorbed. Your body has absorbed the tissue." Weeks later, a final ultrasound showed that the sac was gone as well.
When Linda gave birth to her daughter at 38 weeks, there was no evidence of any kind that another embryo had ever existed—not a thickening in the placenta nor a small fetus hanging by a slender cord from the healthy twin's placenta. If Linda had not kept that early sonogram picture, there would be no way to prove that the vanished twin had ever been conceived. As bewildered as if she'd accidentally misplaced some precious object, Linda believed that her body had reclaimed the dead fetus in order to keep it forever.
Six weeks after the delivery, she came into the clinic for her postpartum checkup. Her daughter, Carolina, was strong and healthy. Would Linda, I wondered, ever tell this child that once in a briny past she had had a sibling? Would this remaining child sometimes sense, as Linda surely must, the presence of the vanished twin?
While I understand much of the biology of the vanishing twin phenomenon, it's the mystery of nature that intrigues me. I wonder how many of us might have once been two, not one.