Understanding Vaginal Bleeding During Early Pregnancy After IVF

Question
I am 34 years old.  I have two daughters who are 13 and 11.  I became pregnant with IVF  on September 18th.  I had an ultrasound on 10/13 and everything looked find except my doctor informed me that there is some blood in my uterus.  I was advised not to worry about it but to contact him if I have any spotting or loss of tissue.  I was also advised to not to have any sexual activity.  To date, I haven't had any spotting, etc.  Can you tell me if this blood is common and if I should be concerned and what the possible causes/effects are?
Thank you so much for your help.

Answer
Usually we do see blood in the uterus in early pregnancy due to "old blood" that has remained in the uterus from a prior period. If it is not interfering with the fetus and you are not showing signs of vaginal bleeding, don't worry about it. The next sonogram will show if it has dissipated.