How To Calculate The Exact Date Of Conception?

i am pregnant.my last period was on 29 march.i had blood test on may 2 it was positive and hcg was 98 and doctor said it was low and said to take test again on may 4 at 1.00pm both were at same time.the hcg was 230 doctor said it was good.she said i am almost 4 weeks pregnant but i thought i was 5 weeks pregnant?how did she calculate that?is my hcg normal?