How Do Some People Not Know How Far Along They Are?

[ballerini]

I have read people saying they were further along than they thought at their dating scan and I wondered how this was possible.
Is this because they didn't remember the date of their last period or is it because the date of your last period is not always an accurate way of working out how many weeks you are?
Curious. Thanks.

OFSTED
Not given pregnancy a thought.

(In the days when we got more notice of an inspection).
When I realized I was pregnant I was 20 weeks.

In fact not sure how I even had the time or energy to conceive. Always joked that he must have 'lifted my nighty whilst I was asleep'!

I had two completely normal looking and feeling periods, plus a random small bleed while I was pregnant. I dated at 20 + weeks at my 12 week scan. It was quite a shock.