I would say its not needed. For one thing, its all an estimation anyway so even a dating scan won't give you a 90% accurate "date" do you remember your last AF and what day you started? Thats the more accurate by far but even then it gives you a few weeks either side. Human gestation is between 38-42 weeks so thinking you are around 4 weeks is fine You could choose a date & stick with that, but you don't have too.