Irregular Periods & Unexpected Positive Pregnancy Test: What to Do

Question
My periods have always been abnormal; I will not have a period for months at a time.  It had not been long since my last period, but I'm not sure when it was because I stopped keeping track. (We were not trying for children. In fact, I thought I was in perimenopause. 37 yo) I had a couple of symptoms of pregnancy--sore breasts and darkened, enlarged nipples--so I tested for pregnancy. It was positive.  I went to my OB/Gyn for a transvaginal US to determine how far along I was.  The US only showed a gestational sac, so the doctor did a hcg quant. My levels were 4180. He said that he expected much lower levels (around 1K - 2K) and that I could be looking at a blighted ovum. He tested again on Friday (I saw him on Tuesday). I will have the results tomorrow or Tuesday. What levels should I be looking at if this is a viable, yet early pregnancy?  From what I've found in my research, 4180 could have been a normal level for me, but the info online is confusing.  What am I looking at?  I have gotten used to the idea of having another baby and am really worried about this.  Thank you for any information you can provide.  

Answer
The range of HCG values make it difficult to pinpoint the age of the baby. A value of 4180 can range from 2 weeks to 5 weeks. A sonogram will not show a fetus and a fetal heartbeat until over six weeks from your last period.   Prior to 6 weeks, we can see an empty gestational sac but no fetus. It would be nice to know the date of the first day of your last period. If you had a value of 4180 on Tuesday, by Friday it should have more than doubled. I would suggest another sonogram in two weeks. if the fetus is over 6 weeks, we will be able to date the pregnancy and should be able to tell the day of ovulation (and conception0. You will also be able to see if the heart is beating.