I just want to add that a lot of people have a much more irregular cycle than they think. Some months, you might not ovulate, but get a period anyway, and as you might have heard, it's fully possible to get your "period" as usual several months into pregnancy. So the only way for sure to know when you conceived is to monitor ovulation, and even then you can't be certain when the egg is actually fertilised as both ovum and sperm can "float around" for a few days.
I wouldn't worry too much about screening, as there still is a pretty wide window. But do mention to the midwife that you're unsure of how far along you are and they might be able to fit you in for a scan earlier than otherwise.
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