How pregnancy weeks are counted
Gestational age is counted from the first day of your last menstrual period, not from conception. That means you are counted as roughly four weeks pregnant when a period is first missed, and two weeks pregnant at the moment of conception.
Weeks are written as weeks + days — "24w 3d" means 24 completed weeks and three days. This notation is standard in maternity records worldwide.
| Trimester | Weeks | What defines it |
|---|---|---|
| First | 0 – 13 | All major organs form. Highest miscarriage risk, and most morning sickness. |
| Second | 14 – 27 | Often the most comfortable. Anatomy scan, and movement becomes noticeable. |
| Third | 28 – 40+ | Rapid growth and lung maturation. Increasing physical discomfort. |
The milestones that matter
| Week | Milestone |
|---|---|
| 4–5 | A pregnancy test is reliably positive |
| 6–8 | First antenatal appointment; dating scan in many systems |
| 10–13 | Combined screening for chromosomal conditions; NIPT available from 10 weeks |
| 12 | Miscarriage risk drops sharply after this point |
| 16–18 | Some people begin to feel movement, earlier in second pregnancies |
| 18–22 | Anatomy scan — a detailed structural survey |
| 24 | Viability threshold; survival outside the womb becomes possible with intensive care |
| 24–28 | Gestational diabetes screening |
| 28 | Third trimester begins; Anti-D given if you are Rh negative |
| 35–37 | Group B strep swab |
| 37 | Term begins; movement monitoring becomes important |
| 39 | Full term |
| 41–42 | Induction typically discussed |
Looking after yourself
- Folic acid. 400 mcg daily from before conception through the first 12 weeks, which cuts neural tube defect risk substantially. Higher doses are advised in some circumstances.
- Nutrition. No extra calories are needed in the first trimester; roughly 340 extra a day in the second and 450 in the third. Protein needs rise to about 1.1 g per kg.
- Avoid. Alcohol entirely, raw or undercooked meat and eggs, unpasteurised dairy, high-mercury fish, and unwashed produce.
- Exercise. 150 minutes of moderate activity a week is recommended for uncomplicated pregnancies, avoiding contact sports and, after the first trimester, lying flat on your back.
- Weight gain. Guidance depends on your starting BMI: roughly 12.5–18 kg if underweight, 11.5–16 kg at a healthy weight, 7–11.5 kg if overweight, and 5–9 kg if obese.
For your due date specifically, see the due date calculator. For nutritional needs, the calorie calculator — though pregnancy targets should be set with your provider.
Frequently asked questions
How many weeks pregnant am I?
Count from the first day of your last period to today, then divide by seven. The calculator does this and shows the result in the standard weeks-plus-days notation.
Remember that gestational age starts about two weeks before conception, so the number is higher than it feels.
When does each trimester start and end?
The first trimester runs from week 0 to the end of week 13, the second from week 14 to the end of week 27, and the third from week 28 until birth.
The calculator dates each boundary for your own pregnancy.
When will I feel the baby move?
Usually between 18 and 22 weeks in a first pregnancy, and often from 16 weeks in later ones, since the sensation is recognisable once experienced.
From 28 weeks, a change in the pattern of movement is worth reporting to your provider the same day.
How accurate is the week count?
It is as accurate as your dating. If your cycles are irregular or you are unsure of your last period, an early ultrasound is far more reliable — within about five days in the first trimester.
If a scan revises your due date, switch this calculator to "known due date" and enter the revised one.
What does "full term" mean now?
Since 2013 the definitions have been narrowed: 39w0d to 40w6d is full term, 37w0d to 38w6d is early term, 41 weeks is late term, and 42 weeks onward is post term.
The change followed evidence that babies born at 39 weeks have measurably better outcomes than those born at 37.