Up and down and up and down and up and down and up.

I bounce my baby sister to sleep, keeping a steady beat on the green yoga ball. The muscles in my legs ache after a while, and my arms are tense from holding her tight against me.

Thump and slap and thump and slap and thump and slap and thump.

My hands make a soothing drumbeat against my sister’s back. I have become an expert at this, gently beating out the rhythms to songs in my head as I woo my sister into sleep’s embrace.

She knows the routine. We do this every night, and though at first she screams and twists in my arms, slowly her cries turn to mewls, and then she is silent. Her eyelids droop. She presses her head against my heart, and for a moment, we merge into the rhythm of coursing blood, the bouncing of the ball, the slapping of my hands against her back.

Together, we form a quiet and predictable rhythm of life.

Up and down and up and down and up and down and up.

The first time I heard my baby sister’s heartbeat, it reverberated from my mother’s swollen stomach, a squelching, static-filled assurance that the baby was more than a figment of the imagination. On the screen in the doctor’s office, I watched the blob of black and white pixels that were my baby brother or sister and tried to see what I wanted to – pink cheeks and blue eyes. As I stared, though, all I could make out was a cosmos of flitting movements. Inside my mother’s uterus, stars were swirling, combining, and shattering. But this galaxy had a heartbeat.

Up and down and up and down and up and down and up.

I can’t keep still as I pace about the tiny apartment. My mother is sick. The tiny, fluctuating galaxy inside her won’t stay. It’s drifting away from us. She is at the hospital, trying to keep the constellations in place. But it’s all fading away. The sky is turning red and the moon is dripping into nothingness. My mother is vomiting out the possibilities to the steady cadence of her own heart in her ears.

“Stay with us,” I whisper into the ether. I am talking to the baby inside my mother. She hears me, and she stays.

Thump and slap and thump and slap and thump and slap and thump.

My suitcase flaps together and I zip it tight. I am going to America. The baby isn’t due for another two months. My mother smiles. She is glowing in the summer light, ripe with my sister, who I still don’t know is a sister. My mother has taken on an ethereal quality, as if she has been impregnated with an angel and it is forcing its way out of her in glowing sparks.

“Good-bye,” I say. I am talking to the baby just as much as my mother.

In America, I go to baby stores and buy things you can’t get in Korea: white onesies and baby powder.

Later, I go to New York to write. Strangely, babies keep creeping into all my best work. Abandoned werewolf puppies and coveted newborn princesses bloom into life on my notebook paper.

I buy my unborn sibling a book or baby shirt wherever I go. Somehow, I feel that I already know the baby.

Up and down and up and down and up and down and up.

I’m back in Korea now, and it is night. My mother cannot sleep. She stands up and paces, then sits down again, only to repeat the process. The baby inside her kicks and twirls. She is dancing a wild dance to the intertwining rhythm of two hearts. Tomorrow the baby will come, my mother says. We are expectant, eager, and a little worried.

The next day, my mother and father leave me with my sisters and brother to go to the hospital. I am full of pure white adrenaline. My fingers snap at the cord of the phone, waiting, waiting for the call.

It does not come.

It does not come.

It finally comes, but my mother still has not had the baby.

We wait. We worry. Finally, we go to sleep. We wake, and the baby is here.

She is a girl.

Thump and slap and thump and slap and thump and slap and thump.

My feet beat against the ground as I run to the car with my father. Awkwardly, we try to install the car seat, but we fail. Eventually, we give up and drive to the hospital.

The baby is not well. Somewhere deep in the darkness of my mother, something went wrong and her heart did not thrive. Instead of growing together, it didn’t form correctly, and now it’s weak and uninspired. I see my baby sister for the first time as we rush her to the heart doctor. She is beautiful, red and puckered like a raspberry. She sucks insistently at my fingers when she finds them with her eager little mouth.

Up and down and up and down and up and down and up.

At home, I watch my new sister’s chest rise and fall as she sleeps. She is so small, and yet her breaths are strong and so well orchestrated she could be a trumpet that someone is blowing air into. Keep breathing, I tell her silently.

The next few months are a blur. She is named. She is driven to doctors hours away. She smiles for the first time. She cries. I discover the soporific effect of the yoga ball on her.

We worry, and then we worry less. A heart surgery might happen, but it can wait. We will see if she can heal herself. Christmas comes and passes. And all the while, we form a new rhythm of life to hers, adjusting the dance of our days to her hunger and her happiness.

Thump and slap and thump and slap and thump and slap and thump.

My little sister is nine months old now, and she is sitting upright in her playpen, opening and closing a pop-up book with tremendous force.

Her heart is strong now. It never needed surgery – she healed herself.

“Is the elephant saying peek-a-boo?” I her, and she flashes a huge grin at me.

I pick her up and she clings to me like the carefree koala bear in her book, adjusting herself over my hip bone.

“It’s time to go to bed,” I tell her.

We sit down on the yoga ball, which has seen better days, but still has its necessary bounce.

“YAHH!” she announces, an obvious protest.

“I will not negotiate,” I tell her, laughing. “You need your sleep.”

She doesn’t agree, but I drape her over my shoulder anyway, and we go up and down, up and down, until her eyes close.

Even when she is asleep, I keep bouncing her, pressing her tiny body close to mine. Together, in this moment, we are united in rhythm, heart to heart.

Up and down and up and down and up and down and up.

Thump and slap and thump and slap and thump and slap and thump.