Since your LMP (gestational age)
Medically your pregnancy will be dated from the first day of your last menstrual period (LMP). This can help estimate your due date.
Full-term pregnancy normally lasts approximately 40 weeks (37–42 weeks).
So in the first 2 weeks, you are not actually pregnant; your body is preparing for ovulation (an ovary releases an egg around day 14 of your monthly cycle).
Fertilisation of the egg by sperm may occur within the next 24 hours or so.
From fertilisation (fetal age)
This is the actual beginning of your pregnancy – your baby's pre-natal age. Full-term is considered to be 38 weeks.
So, when your baby is 3 weeks old, dated from fertilisation, you will be referred to as 5 weeks pregnant – the time of a missed period, when you may just be wondering if you are pregnant.
3 trimesters (since LMP)
Three, 3-month divisions:
1st trimester – 1 to 12 weeks
2nd trimester – 13 to 27 weeks
3rd trimester – 28 to 40+ weeks