Could a Twin Develop?

Jane Foley's picture


Dear Ms Ultrasound,
I am pregnant with my second child right now. Yesterday, at 6 weeks 4 days, I went in for an ultrasound to confirm a heartbeat (yolk sac but no HB detected at 5w4d). The baby looked fine and the HB was detected. Surprisingly they also found another yolk sac in the same amniotic sac, but just a yolk sac, no heartbeat. Our doctor said it's "entirely possible" that the "other" twin will still develop and we'll find out later that we're having twins. How possible is it, really, that one baby would be 6 weeks 4 days with a heartbeat and the other baby would be just a yolk sac? Isn't that a lot of discrepancy? They are obviously identical since they are in the same sac. Also, how common is it to miss a twin on an early U/S. They didn't see the other baby at my 5 week 4 day scan.

Thanks for your help.


Hi Cyndi,
Early pregnancy is such an uncertain time. An early ultrasound can cause much anxiety and uncertainty. In your case you will have to wait a good 10 days to get a solid answer as to how many babies are in the gestational sac.

A 5 week 3 day ultrasound normally does not show a fetal pole. Once you enter into the 6th week, with the proper equipment, a fetal pole should be apparent. Your doctor is correct in saying the second twin may still form. It is within the normal range.Those early ultrasounds always cause anxiety and cause for further investigation into the pregnancy.

I feel it is prudent in many cases to wait further into the pregnancy before doing an ultrasound. This way the ultrasound results will be quite definitive.

With you, all you can do is stay hopeful and positive while you wait for your next ultrasound.

-- Jane, RDMS