I'm 6 weeks pregnant, my progesterone was 28.5 and it dropped to 18.9, my hcg levels are normal still, from 221 to 641. why my progesterone levels dropped? am i at high risk to loose my baby?

Probably not: I'm not sure why you are getting your Progesterone levels checked. The health of the pregnancy is best monitored with HCG levels, which should double every 48 hours, and a few early ultrasounds to document fetal heart beat and fetal growth measures. Btw, your HCG levels at 6 weeks seem a little low. Good luck with this pregnancy!

There is a risk: of miscarriage or ectopic pregnancy please consult your OB

Don't panic: Although the Progesterone drop is a sign of concern in the first trimester, it does not automatically mean you will miscarry, but it does increase the chance. Some OBs would consider placing you on Progesterone therapy to raise your level in hopes of maintaining the pregnancy at least until the placenta is able to take over and produce the needed Progesterone to sustain it. Talk to your doctor.