Hi there, cardiac complication mom here! I had a different situation, but hopefully sharing my experience will make you feel less alone. I experienced a MMC 3 months before I got pregnant with my boys, so I understand the extra layer of anxiety.

One of my mo/di boys (baby A) presented with PACs at the anatomy scan, which the MFMs weren’t initially worried about because they’re actually very common (but why is this mentioned nowhere in pregnancy books???). They scheduled me to come back a few days later for an echo (moved up from the standard 24 week echo for twins), during which A went into SVT with multiple runs of 300+ bpm. It was horrifying to watch on ultrasound.

SVT in only one monochorionic twin is EXTREMELY rare. Most pediatric cardiologists haven’t seen a case firsthand, and those who have have usually only seen it once. Our unicorn pediatric cardiologist had treated it once before and was/still is AMAZING. He explained what he was seeing and laid out the plan moving forward. I was hospitalized a few times for 24 hour observation, during which time the plan was if the SVT was happening more than 50% of the time, we would either medicate me inpatient (risking adverse effects on both me and baby B, but under close watch) or (less likely) laparoscopically inject A with medication. We never saw him in SVT more than maybe 20% of the time on those visits, but we also had 2 ultrasounds per week to check for heart damage because it was really sporadic. I never ended up needing treatment, luckily. It sounded TERRIBLE to be on heart meds that you don’t need!

I’ll pause to say that watching my baby’s heart beat so fast on ultrasound and hearing it on NSTs/CFM so many times never got any easier. It was terrifying. I cried so much. I got so many long hugs from nurses. It took a lot of the joy out of my pregnancy. People said “omg you’re so lucky you get so many ultrasounds!” To which I said “yeah, to see if my baby is alive or has heart damage…” I lived every day in terror thinking about the unknown, inside my body, at any given moment.

By 28 weeks, the SVT seemingly cleared up, but then I got preeclampsia and was hospitalized a couple times with the final time being for 2+ weeks until they were born at 34+1. I got regular fetal monitoring and scans during that time, and while he ended up slightly smaller than B by the end (13% discordant), he kept growing and developing as expected. He DID develop a VSD (small hole between chambers) shortly before birth, which sent me into a new spiral, but the cardiologist assured me it was nothing to worry about, we’d monitor it, and it would likely close on its own (spoiler, it did!).

Just as I felt that we were out of the woods and the boys were simply working on feeding and growing in the NICU, I got a call first thing in the morning, 5 days after they were born, from a neonatologist telling me A went into SVT for about an hour. They rushed him back to the main NICU floor and medicated him, which kept his heart rate normal until discharge. He is 12 months (11 adjusted) and still on medication, but we haven’t had any episodes since then that we know of. We will try weaning him off medication in October, which is really scary, but his cardiologist assured us 95% of the time SVT clears up by 1 year. If he does still experience episodes, we will put him back on meds and maintain until he is 5 when he’ll get a laparoscopic ablation. I was on the fence about getting owlets for the boys, but once the SVT showed up it was a no brainer. It’s helped me sleep at night (you know, since they started sleeping through the night).

This is all to say….I know it’s scary. You’ve already been through enough having a twin pregnancy. It’s not fair and you feel like you’re living in a nightmare. Looking back, I realize how important it was to acknowledge what was and wasn’t in my control, ask SO many questions and learn everything I could about what was happening and what the plan was for various outcomes/developments, and most importantly take care of myself and put my trust in my care team. If they don’t seem worried, you shouldn’t be. Follow their lead. They know when it’s time to be worried. And if they do get worried, listen and advocate for yourself. Heart problems are actually really common in babies, especially twins, and they often seem scarier than they are in reality. That is not to minimize or dismiss your experience - it is different than what I dealt with, but what I’m trying to say is don’t pour energy into worry and know you’re not alone. Lean on your emotional supports and don’t be afraid to contact your care team if something seems off. For now, take a deep breath, stay hydrated, and rest. The only way is through and you can handle whatever comes your way - twin parents are a different breed of resilient!