When I became pregnant through fertility treatment, I had something in common with about 30 percent of other women who undergo IVF or IVF with donor eggs and receive two or more 5-day blastocysts in utero. I was pregnant with twins.

I suspected this was the case from the outset. I was tracking my hCG numbers, and they were rising too quickly for there not to be twins. The numbers are supposed to double every two days or so; mine instead were tripling and quadrupling. So when we went for our first ultrasound at five weeks, I was not at all surprised when the ObGyn poked around with the ultrasound wand and spotted not one, but two tiny, flickering heartbeats.

My husband, Paolo, was in the exam room with me. “Two?” he said in disbelief as I just smiled smugly. I knew there would be two; I just knew it. He told me later that he nearly took a header and hit the floor, so great was his shock at the news.

Despite my suspecting we’d have twins, it was still a lot to take in for me, too. After our visit, we both sat in the hospital lobby for several minutes, trying to digest the news. Twins. After a few years of struggling with my infertility, our family would be complete, just like that. But we’d need a twin stroller, two cribs, two car seats, two of everything. I started to rattle off the list and Paolo said, wisely, “Not yet, Liz.”

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We announced to Paolo’s family and mine that we had two babies on the way. I researched online about multiples and high risk pregnancy—at 45 a single pregnancy was risky enough for me; with twins, I needed to be in the hands of a high-risk OB specialist. So I searched online for the closest doctors. And every now and then, I’d sneak a peek at those twin strollers.

About 8 weeks into the pregnancy, before our first appointment with a specialist, I woke in the middle of the night with what felt like either intestinal or menstrual cramps. I went to the bathroom and saw that my underwear was stained with blood. I called to Paolo, who ran to the bathroom. I’ll never forget the look on his face—absolutely stricken.

We went to the emergency room, expecting an ultrasound to reveal that the pregnancy was over. The doctor pushed the wand around inside me, and located one empty embryonic sac that had previously held that little heartbeat. My heart sank, and I think I felt Paolo’s hit the floor too. The doctor prodded some more, and then located one of the best things I’d ever seen in my life. The other embryonic sac was very much still occupied, by a miniscule fetus that looked like a cherub running in place. We still had one. We were still pregnant.

I only cried one time about that twin we lost. Maybe because it was so early in the pregnancy. Maybe because we were scared and preoccupied with not losing the remaining embryo. And maybe because we both felt so lucky, especially as the weeks passed and we crossed out of the first trimester danger zone, that I was experiencing a healthy, complication-free pregnancy

It’s been more than two years since that trip to the emergency room. We have an active toddler who is running us both ragged. We’re about to embark on potty training and giving up the pacifier. Still, I think about that twin we lost, and the “what ifs?” What would it have been like with two? Would it have been a boy or a girl?

The thinking is that when one twin dies in utero, it does so that the other one can survive, sort of like the weak member of the pack sacrificing itself for the sake of the strong. Some people think that it’s God’s hand in things, preventing a pregnancy that would result in a sick baby who struggled and failed to survive. Others say it’s just fate, that every miscarriage is a life that just wasn’t meant to be.

I tend to side with science, and just accept that the second embryo was not healthy and viable. Better that we lost it sooner, rather than later, right? But I admit that two years later, when I see couples with twins, I still feel a pang of jealousy and loss. Why them and not us? Especially now, as we long for our daughter to have a sibling and we grapple with the fact that we simply can’t afford another fertility treatment. Why couldn’t that twin have just hung on?

I know we’re among the lucky ones. We have a beautiful, healthy daughter. She came to us after just a few rounds of fertility treatments, not after the years of heartache, expense and disappointment that so many couples endure, only to wind up empty-handed still. I know I have no right to complain about having just one child. And still I find myself asking, “Why couldn’t that twin have just hung on?”

When Naomi is old enough to understand, we’ll explain to her how she was conceived. Maybe we’ll tell her about her twin sibling that wasn’t meant to be, or maybe we won’t. I don’t know what’s best. Life doesn’t give you an instruction manual for these kinds of decisions, though that sure would be nice. And until the day comes that we decide to tell her or not tell her about her twin, I’ll feel lucky every time I hear her giggle or babble or call out our names. But I’ll always wish she had a brother or sister to play with, too.