Most of us think of pregnancy as lasting nine months, but health care professionals usually refer to pregnancy in terms of weeks. Most pregnancies are said to last 40 weeks, although delivery usually happens anywhere between 38 and 42 weeks. To calculate your due date, count forwards forty weeks from the first day of your last period.
When you're trying to work out how far into a pregnancy you are, count the weeks from the first day of your last period, not from when you think you conceived. So, when you are twelve weeks pregnant, you might only have conceived ten weeks ago. It feels strange to work it out like this, but that's how your health carer will work it out too