Is it really possible?

Andrea • Married June 2012. Rainbow Baby born May 2017. Due with #2 January 2022.

At my 8 week appointment the baby measured 6 weeks... is it possible considering the following?

9/17 First day of last period

9/29 Unprotected sex

10/17 Positive HPT, First Response Early result

11/12 Ultrasound measures baby 6 weeks instead of 8

Before I started my birth control I did have irregular periods.