The short answer: a pregnancy calendar divides 280 days from the first day of your last period into forty seven-day weeks and puts a real date against each one. The first trimester runs to the end of week 13, the second to the end of week 27, and the third to birth. The appointments that fall inside it are, in the usual international windows: booking at 8–10 weeks, dating scan at 10–14, first-trimester screening at 11–14, anomaly scan at 18–21, glucose tolerance test at 24–28, and anti-D at around 28 weeks if you are Rh-negative. Full term begins at 37 weeks, and anything up to 42 weeks is a normal length of pregnancy.
Three ways in, one calendar out
All three tabs above compute the same thing — the first day of the last period, or its equivalent — and everything else follows from it:
- From your due date. Subtract 280 days. Use this if you have had a dating scan, because the date it gave you is more accurate than anything a cycle-length adjustment can produce.
- From your last period. Enter the real cycle length rather than assuming 28 days: a 32-day cycle means ovulation four days later, so the whole calendar shifts four days later too.
- From how far along you are. Useful when you have been given a figure like "12+3" at an appointment but nobody wrote the due date down.
Nothing is sent anywhere. The whole calculation happens in your browser, which is also why the print button produces a clean PDF rather than a download from a server.
What happens when — the antenatal schedule
These are the windows most health systems work to. Your own schedule will differ in the details, and a country with a nine-visit pathway looks different from one with fourteen — but the biological reasons for the timing are the same everywhere, which is why the windows themselves barely move.
| When | What | Why then |
|---|---|---|
| 8–10 weeks | Booking appointment, first bloods | Early enough to plan care and screening, late enough to confirm the pregnancy |
| 10–14 weeks | Dating scan | Crown-rump length dates the pregnancy most accurately in this window |
| 11–14 weeks | Combined screening | Nuchal translucency is only measurable while the fetus is this size |
| From 10 weeks | NIPT, if chosen | Enough fetal DNA is circulating in maternal blood by now |
| 18–21 weeks | Anomaly scan | Organs are formed and large enough to examine, and still easy to image |
| 24–28 weeks | Glucose tolerance test | Insulin resistance peaks here, so this is when gestational diabetes shows |
| ~28 weeks | Anti-D if Rh-negative; repeat bloods | Prophylaxis before the third trimester; anaemia is commonest now |
| 16–32 weeks | Whooping cough vaccination | Maternal antibodies cross the placenta and protect the newborn |
| From 28 weeks | Daily kick counts | Movement patterns are established enough to notice a change |
| 35–37 weeks | Hospital bag, birth plan, position check | Before the point at which labour would not be stopped |
| 37–42 weeks | Watch for signs of labour | Full term; induction is usually discussed around 41–42 weeks |
How the forty weeks are grouped
First trimester — weeks 1 to 13
Weeks 1 and 2 precede conception entirely; the embryo exists from week 3. Every major organ system is formed by the end of week 12, which is why folic acid matters from before the positive test rather than after it. Symptoms tend to peak around weeks 8 to 10. See the first trimester guide.
Second trimester — weeks 14 to 27
Nausea usually settles, energy returns, and the anomaly scan falls in the middle of it. First movements arrive somewhere between 16 and 24 weeks — later in a first pregnancy. See the second trimester guide.
Third trimester — weeks 28 to birth
Appointments get closer together, kick counting begins, and the practical preparation happens. From 37 weeks the pregnancy is term and the plan shifts from "keep going" to "watch for labour". See the third trimester guide.
Dates that are not on any calendar
Contact your midwife or maternity unit the same day, whatever week the calendar says you are in, if you have:
- Reduced or changed baby movements after 28 weeks
- Vaginal bleeding at any stage
- A severe headache, visual disturbance, or sudden swelling of the face and hands — the signs of pre-eclampsia
- Fluid leaking from the vagina, or regular painful tightenings, before 37 weeks
- Severe abdominal pain, a fever, or pain on passing urine
None of these waits for the next scheduled appointment.
Not sure of your date yet? The due date calculator works it out from your last period, a conception date or an IVF transfer, and the conception date calculator runs the same arithmetic in reverse. If you are not pregnant yet, the ovulation calculator is the one to start with.
Related tools and guides
FAQ — The pregnancy calendar
How does a pregnancy calendar work?
Pregnancy is dated as 280 days — 40 weeks — from the first day of your last menstrual period. Give the calendar any one of three things (your due date, that first day, or how many weeks along you are today) and it works out the other two, then assigns a seven-day date range to each of the forty weeks and marks where you are now.
When are the main scans and tests?
The usual international windows are: booking appointment at 8–10 weeks, dating scan at 10–14, first-trimester screening at 11–14, the anomaly scan at 18–21, the glucose tolerance test at 24–28, anti-D for Rh-negative women around 28, and whooping cough vaccination from about 16–32. Exact timing varies by country and by your own care plan — the appointment schedule guide goes visit by visit.
Why is my calendar two weeks "ahead" of conception?
Because dating runs from the last period, not from conception, and ovulation is roughly two weeks after that. It means you are counted as 2 weeks pregnant on the day of conception and 4 weeks when a test first turns positive. The conception date calculator explains the offset and works backwards through it.
What if my cycle is not 28 days?
Use the "from your last period" tab and enter your real cycle length. A longer cycle means later ovulation, so the whole calendar shifts later by the difference; a shorter one shifts it earlier. If you have already had a dating scan, use the due date from that instead — it is more accurate than any cycle-length adjustment.
Can I print the calendar?
Yes. Fill it in, then use the print button under the results — it prints or saves as PDF through your browser, with all forty rows and your own dates. Nothing is uploaded anywhere; the whole page runs in your browser.
How many months is 40 weeks?
About 9.2 calendar months, which is why the traditional "nine months" never quite lines up. Pregnancy months are conventionally grouped as 4–4–5 weeks rather than by the calendar, and that is the grouping the Month column here uses.
When does each trimester start and end?
The first trimester runs to the end of week 13, the second from week 14 to the end of week 27, and the third from week 28 to birth. Full term is from 37 weeks; anything from 37 to 42 weeks is a normal length of pregnancy.
Will my due date change?
It can, once. If the dating scan disagrees with the period-based estimate by more than about 5 to 7 days, the scan date is adopted — and then it is fixed for the rest of pregnancy, even if later scans measure the baby big or small for dates. Come back and re-enter the new date and the whole calendar moves with it.
Sources
- ACOG — Routine Tests During Pregnancy
- NHS — Your antenatal appointments
- NHS — Screening tests in pregnancy
The pregnancy calendar — at a glance
A pregnancy calendar splits 280 days from the first day of the last menstrual period into forty weeks and puts a real calendar date against each one, which turns a single due date into a plan. The first trimester ends with week 13, the second with week 27, and the third runs to birth; full term starts at 37 weeks and anything to 42 weeks is a normal length. The appointments inside those weeks cluster at predictable points because the biology does: booking and first bloods at 8–10 weeks, the dating scan at 10–14 while crown-rump length is still the most accurate measure of gestational age, combined screening at 11–14 while nuchal translucency is measurable at all, the anomaly scan at 18–21 when organs are formed and still easy to image, the glucose tolerance test at 24–28 when insulin resistance peaks, and anti-D around 28 weeks for Rh-negative women. Kick counting starts at 28 weeks, the hospital bag and birth plan belong to weeks 35–37, and labour is watched for from 37. A due date can change once, if the dating scan disagrees with the period-based estimate by more than five to seven days — after that it is fixed. Pair the calendar with baby size week by week to see what each of those dates actually looks like.