Hello,
 In the current scenario, the possible suggestions are as follows: 
1. If it is a non-viable twin, usually absorption occurs as 
pregnancy progresses or the bulge shrinks in size. 
The serum beta-
hCG titres, if charted, would show a fall, previously being in the range of a twin pregnancy. 
2. A mass usually increases in size as pregnancy advances depending on its nature. A high resolution scan is helpful to clarify. 
3. Thickened and folded endometrium is a common occurrence and resolves in course of time.
 A follow-up sonogram after a week suffices. 
Endometriosis usually does not progress during pregnancy owing to the high 
progesterone levels and suppressed ovulation. 
Please get a follow-up sonogram and plan further management.
Hope I have answered your query. Let me know if I can assist you further.
Take care
Regards,
Dr Shanti Vennam, OBGYN