Second Sac Increases Risk of Miscarriage

Jane Foley's picture


Dear Ms Ultrasound,
My name is Royann and I am just about 7 weeks pregnant. I just had an ultrasound and the doctor said that the baby looked great and had a strong heartbeat, but just a couple inches away from the baby is another sac that looked like it had some fluid in it and maybe a yolk sac.

The doctor didn't know if this was a subchrionic hematoma or if it was going to turn into another baby. I have another ultrasound in two weeks. He said that my chances of having a miscarriage are greater. Could you please tell me why that is?

Would another baby be growing even though one is at 7 weeks? Any information would help.


In the last decade or so, with the advent of higher tech ultrasound, what has been discovered is that twins, happen much more commonly then anyone knew. This has presented ambiguity, with early diagnosis on an ultrasound. What commonly happens is, two eggs implant in the uterus, but only one continues to grow normally, leaving what looks like a blood clot in the uterus next to the growing gestational sac.

The difficult part for the sonographer and the radiologist, is when an ill-defined portion is seen in the pregnant uterus, with a normal gestational sac, differentiating a blood clot/hematoma/subchorionic bleed, vs. a second/twin sac that did not make it, is sometimes impossible.

When a finding like yours is discovered, you will have serial ultrasounds to monitor your pregnancy, and make sure that the area of concern next to the gestational sac is resolving.You are at a slightly higher risk for miscarriage because if the body thinks it want to expel the non-viable sac, the uterus will start to contract, thus putting the second baby at risk. Please note we follow these kinds of findings all the time and watch them go to a normal full term pregnancy.

From what you have described to me, it sounds like this is a second sac. You will know for sure on your follow up ultrasound if it is a viable sac or not. It is not likely, since your pregnancy is already 7 weeks along.

-- Jane, RDMS