Early Pregnancy Concerns: Spotting and Positive Test - Seeking Advice

Question
LMP was 6/6/06, which I thought was normal.  My husband and I have talked about wanting another and in previous two months tried, but did not think this month was possible because I would be traveling during what I thought was my ideal time.  I left on a family vacation (without husband) on 6/19/06, and started spotting dark brown, on 6/24 I got a positive pregnancy test. ?  I had cramping and heavier spotting, some bright red, some dark red when I wiped and some small amounts of matter. I flew home on the 27th and went straight to the ER.  They did HCG which came back in the 700's, and an ultrasound which showed no pregnancy in uterus.  Went back to the OB's office today who did another intravaginal ultrasound which showed the same.  Am I too early to show anything yet and still have possibility of viable pregnancy while having light bleeding/mild cramping?  Have two healthy children already age 6 & 3, with no history of this with those pregnancies. Doctor has ordered me to have another HCG and I am wondering if I should wait maybe until Friday a.m. to give it the best chance to increase.  She has also ordered an ultrasound with possible D&C to be scheduled as she is doubtful. Any info you can share would be extremely appreciated.

Answer
Prior to the Beta HCG reaching 1000 (the discriminatory zone) we do not see any fetus in the uterus.) We may sometimes see a gestational sac, but no baby. This corresponds to a 6 weeks size fetus (from your last period).  I would wait another week and take another sonogram and HCG test before having a D&C.