Unexpected Pregnancy After IVF: Ultrasound Concerns & Early HCG Levels

Question
Hello Dr.
I am 41yo and diagnosed with high FSH (19 so not THAT high) and I have no children.  I didn't start trying to conceive until 1 year ago.  Went thru a failed IVF cycle in October and began menses on Nov. 11 (+/- 1 day).

Took a HPT on 12/10 with a positive result.  Wow, I got pregnant naturally - I was not expecting that. I went to IVF clinic for blood test - positive.

12/14 HCG = 1572
12/15 HCG (21 hours later not 24) = 2300

On 12/15 which should be approx. 5 weeks they could not find a gestational sac thru vaginal ultrasound.  HCG seems to be going up well so I am confused.

I go back tomorrow for another HCG and ultrasound but do you have any ideas what might be going on?

Answer
Until 6 weeks gestation (from the first day of your last pereiod) a sonogram will not show a fetus. Between 5-6 weeks, we can sometimes see an empty gestational sac. After 6 weeks, we usually see a fetus and a fetal heartbeat.If your last period started on November11, you are now only 4 weeks 6 days from your lst period. I would repeat the sonogram in 2 weeks. By then, there should be a fetus, a sac, and a fetal heartbeat seen on sonogram. Be patient. The HCG values appear to be rising nicely.