Fetal heart rate 72bpm and CRL .42cm at 8 wk gestation

Hi,

I am going to be 41 years old in August 2012. I am currently pregnant at 8 wks and 5 days gestation from LMP.  I had my first OB appt at exactly 8wks. The fetus measured 6.2 wks at .42 CRL with 72 bpm heart rate. But the nurse said its showing 5.6 wks and the heart just started beating which is why its at 72 bpm. She said that it would catch up and asked me to come back in 2 weeks for another u/s.
I had a miscarriage in January at 7 wk 6 days gestation. My first OB appt then was at 7 wk and 1 day when my fetus measured .66 cm CRL with around 78 or so bpm with 6 wk and 5d.
I was having some cramps for the last 3 or 4 days with no morning sickness, but since yesterday I have been having morning sickness again.

I dont know what to make out of this. I was charting my bbt and ovulation so I am pretty sure I ovutated between CD  16 and 18. My LMP is may 30th. So I am really concerned on why the fetus measures so low at 8 wks.
I have a 2 year old son. When I went for his fist OB appt at 7 wks and 3d gestation, he measured .79 CRL and  around 145 bpm.

I compared the all 3 u/s images. For the first 2 the pole size is almost filling up the gestational sac. but for this 3rd one the pole looks like a dot in the sac. The sac looks much bigger this time.

Please help me in understanding this. Can the fetus catch up and measure normal as it should e.g .8 cm CRL with a strong 150 bpm in my followup u/s. If it does, would it be a healthy as it grows or would it have any chronic issues throughout its life.

Seeking help, and going crazy.

Thanks so much