€49Women's Health Consultation
Confidential women's health assessment with a doctor registered with the Portuguese Medical Association. Contraception, hormonal health, menopause, PCOS, and more, by secure video call.
20 min
Free health tool
Due date calculator Portugal
Enter the first day of your last period to see your estimated due date, how far along you are today, and when each trimester starts.
- Adjusts for cycle lengths from 20 to 45 days
- Trimester dates and the 37–42 week term window
- Runs in your browser — nothing is sent or stored
Your due date
The day bleeding started, not the day it stopped.
days
Days from the start of one period to the start of the next. Leave it at 28 if you are not sure.
08/10/2026
This is an estimate from the dates you enter, not a diagnosis. A dating scan measures the baby directly and replaces it.
01
Your pregnancy week by week
Pregnancy is dated from the first day of your last period, not from conception, and it runs for 40 weeks from there. Before 37 weeks is preterm, 42 weeks and over is post-term, and the weeks in between are when almost every baby actually arrives.
The table below is the whole 40 weeks, split the way maternity services split them.
1 – 13
- Stage
- First trimester
- What usually happens
- Antenatal booking, first blood tests and the dating scan
14 – 27
- Stage
- Second trimester
- What usually happens
- The anomaly scan, normally around 20 weeks
28 – 36
- Stage
- Third trimester
- What usually happens
- Growth, blood pressure and position checks come closer together
37 – 38
- Stage
- Early term
- What usually happens
- Labour from here on is no longer preterm
39 – 40
- Stage
- Full term
- What usually happens
- The estimated due date sits at the end of week 40
41
- Stage
- Late term
- What usually happens
- Monitoring increases and induction is usually discussed
42 and over
- Stage
- Post-term
- What usually happens
- Birth is normally brought on rather than waited for
02
How the due date is calculated
The estimate is Naegele's rule: 280 days, or 40 weeks, from the first day of your last period. That assumes a 28-day cycle with ovulation around day 14.
If your cycles are longer or shorter, ovulation moves with them, so the calculator shifts the date by the difference. A 32-day cycle puts the estimate four days later; a 24-day cycle brings it four days earlier.
Worked example: a last period starting on 1 January with a 28-day cycle gives 8 October. The same period with a 32-day cycle gives 12 October.
03
Why a dating scan replaces this estimate
A calculator can only work with the dates you give it. A scan measures the baby, which is why its date is the one your maternity team keeps using afterwards.
- It depends on remembering the first day of your last period accurately.
- It assumes you ovulated in the middle of your cycle, which not everyone does.
- Irregular cycles, recently stopped contraception and breastfeeding all make the last period a weak anchor.
- A first-trimester scan measures the baby, usually between about 11 and 14 weeks, and is accurate to within a few days.
- After IVF the dating comes from the transfer date rather than a period, so this estimate does not apply.
04
When to contact a doctor or midwife
- You think you are pregnant and have not yet arranged antenatal care — the first appointment and the dating scan both belong early.
- You have bleeding, cramping or pain on one side in early pregnancy.
- You have severe or constant vomiting, or cannot keep fluids down.
- You have a long-term condition or take regular medicines — some need reviewing as soon as a pregnancy is confirmed.
- You have reached 41 weeks, or you are past your due date and the baby's movements have changed.
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Due date calculator — FAQs
How accurate is a due date calculator?
The arithmetic is exact; the assumptions behind it are not. Only about one pregnancy in twenty ends on its estimated date, and the great majority of births happen in the weeks either side of it. A first-trimester scan is the accurate version, and once you have had one, its date is the one that is used.
How many weeks pregnant am I?
Pregnancy is counted from the first day of your last period, not from conception — which is why you are already around four weeks pregnant when a test first turns positive. Conception happens roughly two weeks after that first day, so the pregnancy is always dated about two weeks 'older' than the embryo.
What does 9w 3d mean?
Nine completed weeks and three days since the first day of your last period. Maternity records are written this way because a whole week is too coarse when a scan or a test has to happen inside a particular window.
Can I use this if my cycles are irregular?
You can, and entering your usual cycle length will get you closer than the 28-day default. But an irregular cycle is exactly the case where the last period is a weak anchor, so treat the result as a rough guide until you have been scanned.
Does the due date change after a scan?
Often, yes — usually by a few days. The scan measures the baby instead of counting from a remembered date, so it is the one that is kept. A change of a week or more is common when the date of the last period was uncertain.
Does this due date calculator store my details?
No. The calculation runs entirely in your browser. Nothing you type is sent to our servers or saved.
How do I arrange antenatal care in Portugal?
Through your local health centre, where the maternal health consultation follows a low-risk pregnancy and links in with a hospital when that is needed. The first blood tests and the scan are arranged there, and the scan's date is the one your pregnancy record uses afterwards.
When is the dating scan done in Portugal?
The first-trimester scan is usually done between about 11 and 14 weeks, with the anomaly scan around 20 weeks. Until then the estimate above is what you have — and our registered doctors can talk through early symptoms or medicines you already take by video.
Questions in early pregnancy?
Talk to a registered doctor by video about symptoms, the medicines you already take, or anything you would rather not sit on until your first appointment.