Partner a: ongoing sex. partner b: i had sex one time october 13, 5 days later took a plan b. nov 6 my fetus measured 7w5d. is partner a the father?

Maybe. Just count...: One can count backwards 5 weeks and 5 days, if the baby fetus is now 7 weeks and 5 days, to find the estimated conception date. The standard way of counting is: conception is 2 weeks AFTER the official start of the pregnancy. That means if pregnancy officially started 7 weeks ago, conception was 5 weeks ago. All dates are estimates, plus or minus one week. That's right, all dates are estimates.