(>30 yrs old) is always a high risk pregnancy with various complication and miscarriage
is one of them.
Your LMP was Oct 20th and on 7th wk scan,it reveals yolk sac 2.7mm & indicates late pregnancy.
Dark brown/black discharge may indicate early abortion/implanted bleeding and need to confirm with USG scan after 2 wk to assess fetal development & viability.
In the mean time,you take folic acid
with natural micronised progesterone
to prevent miscarriage some what but not always 100%.
Avoid heavy work/long journey/stepping uneven surface,take protein rich diet,complete bed rest, avoid stress and maintain personal hygiene.