Threatened abortion and baby measures 3 weeks small

So, I know this will be long, but I want to explain the whole situation...This will be my 3rd child, and I'm 30. My LMP was around 7/24. I had heavy bleeding around 8/14, that I actually thought was a period at the time. I didn't realize I was pregnant. It was very heavy, lasting around 5 days, but no extreme cramps. After this bleeding, I want to stress, my DH and I had NO sex for the next month (sadly enough, stressful month- LOL) So, when I missed my period in Sept, I was absolutely SHOCKED to get a pos. pregnancy test!!! I knew that it had to have actually been from the month before. Went to the Dr., had an u/s, and the baby had a heartbeat and measured 5 weeks 5 days. Now, I know that this very coincidentally matches the dates IF the Aug. bleeding was a period, BUT let me stress again...there was NO sex that month, it's just not possible... SO, fast forward to last Tuesday... and I suddenly start having some light cramps and start bleeding heavily. I actually thought I had lost the baby, it was not only bright red blood, but there was tissue as well (although, not large clumps...) Went to the Dr again. Had another u/s and surprisingly the baby was still there with a heart rate in the 160's and measuring 10 weeks 5 days. Bleeding lasted about 5 hours and spotted brown for a couple of days after that. The Dr. says everything looks fine, but I'm worried about the fact that the baby measures around 3 weeks behind what it should- even though the growth seems to be at a steady rate. Anybody have any insight? I'm still worried about possibly losing it since this is my 2nd threatened miscarriage, and the baby measures so far behind...

[Dr. Kokil Mathur]

Hi! Well obviously if there was no sex in the month then you conceived a month earlier—unless your memory is playing games (less likely but not unlikely). However, if you have repeated miscarriage, its bets to undergo tests to rule out fetal abnormality, genetic causes, Rh blood group incompatibility, low progesterone hormone, infection of the uterus, endometriosis, fibroids, etc  are some causes for early abortion. Usually early in pregnancy the duration of pregnancy by LMP (last menstrual period) and ultrasound method is similar—maybe a week apart. So I am more inclined toward your forgetting the date of intercourse in the previous month. However, if the baby is growing normally, then just try find the cause of repeated abortions you had, and consult your gynecologist regarding this.
Take care!