My hcg levels were 143 on the 28th 30 hours later they were 136 does this automatically mean miscarriage?

HCG: HCG doubles every 2-3 days until reaches its max on wk 8-11, so it shouldn't come down , I would repeat it again to make sure this is not a lab error, if still low then please talk to your OB doctor.

If HCG goes down..: Usually for a viable pregnancy HCG levels should be coming up, at 48hrs being double the value than the prior. If levels are coming down it might be possible that either there is an empty ovum(egg), or the pregnancy is not progressing. In any case please make sure you follow up with your GYN provider because there is always possible to have false values and might need to repeat to confirm.

HCG not rising: Human chorionic gonadotropin (HCG) is produced in pregnancy, and rises very quickly, even over a 30 hour time frame, so any lowering from one measurement to another is not usual for normal pregnancy Some tests are not always accurate (lab error) though, so test again in 5-7 days. If no increase, generally the pregnancy will miscarry.