Hi, My Lmp Was February 9th And I Had Sex

Hi, my lmp was february 9th and I had sex once on the 25th due to hubby being away. I found out I was pregnant on March 7th and went for a scan yesterday where all they saw was a gestational sac of 14mm. My hcg levels were 20,000 and I have irregular periods ranging anything up to 44 days. They said the sac looked a good shape and was in the correct location and the progesterone was good around it. Should I be seeing a yolk sac by now though?