Twins different sizes at 8 weeks

Hi, im 27 and this is my 3rd pregnancy i have 2 healthy children. I went for a scan the other day and was told im expecting non-identical twins. a strong heartbeat was seen on both twins. however they measured significantly differnet one measuring 8 weeks 4 days the other measuring 7 weeks 5 days. I was advised about an anencephalic twin and also about vanishing twin syndrome and booked for a re-scan in 3 weeks time.
I have since read about both these conditions on the internet and am greatly concerned. Please can you advise me why I have been warned of these two conditions and the chances of me getting/having them. Also I felt reassured both had a strong heartbeat but now am very worried, does the heartbeat make a difference. Thank you

[Yao Sun, MD, PhD]

As a neonatologist, I do not have the knowledge to answer your question fully.  You should ask your obstetrician, or ask for a referral to a Perinatologist.

Big discrepancies in size at this early gestational age may indicate a risk for congenital anomalies, and that is why you were told about the possibilities you listed.