I just learned

that within about three days after conception, the fertilized egg starts dividing very fast into many cells and passes through the fallopian tube into the uterus, where it attaches to the uterine wall. The placenta, which will nourish the baby, also starts to form. So it takes 3 days for the fetus to actually start growing after conception once it's attached to the uterine wall. 
I find that interesting because my baby was measuring 6w5d at my ultrasound and before I went into my ultrasound I assumed I was 7w1d so it seems to me that I conceived baby on May 2 but it didn't actually start growing until May 4 or May 5 once it finally attached itself to the uterine wall, does that make sense to anyone? Because I know at an ultrasound they measure the baby and set the expected due date by that depending on babies size.