Pregnancy Calculator: Due Date and Week of Pregnancy
The first question after a positive test is always the same: when is the due date and how many weeks am I. The answer does not depend on the day you found out, but on the first day of your last period, because pregnancy is by convention counted from that day, two weeks before conception even happened. The calculator below gives your expected due date, the exact week and day you are in today, the trimester, and the dates of the most important check-ups during pregnancy. Below the calculator you will find how the due date is worked out, how reliable it is, and when a gynaecologist changes it.
Pregnancy calculator
The due date is an estimate. Your gynaecologist confirms or adjusts it from a first-trimester ultrasound.
How the due date is calculated
Pregnancy lasts 280 days on average, that is 40 weeks, counted from the first day of the last period. This is Naegele's rule, used in obstetrics for almost two centuries: add seven days and nine months to the first day of your last period. The calculator does the same, precisely in days, with one correction the rule ignores.
That correction is your cycle length. The rule assumes a 28-day cycle with ovulation on day 14. If your cycle is 32 days, ovulation happened about four days later, so the due date moves four days forward. With a 25-day cycle it moves three days back. That is why the calculator asks for cycle length: without it, the due date for women with longer or shorter cycles is off by a week or more.
If you know the exact date of conception, for example from ovulation tracking or after IVF, choose that option. Then 266 days are added to the conception date, and the weeks of pregnancy are still counted so that conception falls in week two, because the whole counting system in obstetrics is based on menstrual age.
Weeks, months and trimesters
The most common confusion is about counting weeks. When a doctor says you are in week 12, it means 11 weeks and a few days have been completed, just as you are in your first year of life until you turn one. Reports therefore use completed weeks plus days, for example 11+4. The calculator shows both forms.
| Trimester | Weeks | Months | What marks it |
|---|---|---|---|
| First | 1 to 13 | 1 to 3 | Organ formation, nausea, first ultrasound and screening |
| Second | 14 to 27 | 4 to 6 | First movements, anomaly ultrasound, gestational diabetes test |
| Third | 28 to 40 | 7 to 9 | Rapid fetal growth, preparation for birth, CTG |
How accurate the due date is
Only about five percent of women give birth on the due date itself. The due date should be read as the middle of a period rather than a fixed day: birth between the completed 37th and 42nd week counts as at term, and most women give birth within two weeks before or after the calculated date. First pregnancies run late a little more often, later ones a little more often come early.
The most reliable way to date a pregnancy is an ultrasound measurement of the embryo between weeks 8 and 13, which is accurate to within a few days. If the date from a first-trimester ultrasound differs from the period-based date by more than a week, the gynaecologist corrects the due date to the ultrasound and only that date is used from then on. Later ultrasounds have a larger margin of error, so the due date is not normally changed after week 20.
The calculator gives a wrong result in three situations: when cycles are irregular so the day of ovulation is unpredictable, when you conceived right after stopping hormonal contraception, and when the bleeding you took for a period was actually implantation bleeding at the start of the pregnancy. In all of these an early ultrasound is the only reliable answer.
What is done in which week
The dates in the calculator are approximate windows for the check-ups done in every normal pregnancy. The order is always the same; the exact schedule is set by the gynaecologist following the pregnancy, depending on your results and any previous pregnancies.
| Weeks | Check-up | Why it matters |
|---|---|---|
| 6 to 8 | First ultrasound | Confirms the pregnancy is in the uterus, shows the heartbeat and number of embryos, first check of the due date. |
| 11 to 14 | NT ultrasound and double test | Nuchal translucency measurement with a blood test estimates the risk of chromosomal anomalies. Only possible in this window. |
| 20 to 24 | Anomaly ultrasound | Detailed examination of fetal anatomy, placenta and amniotic fluid. |
| 24 to 28 | OGTT | The glucose tolerance test detects gestational diabetes, which often has no symptoms at all. |
| 28 to 32 | Growth ultrasound and Doppler | Checks that the baby is growing in line with the weeks and how blood flows through the placenta. |
| 35 to 37 | GBS swab | Vaginal and rectal swab for group B streptococcus decides whether antibiotics are needed during labour. |
| from 36 | CTG | Records the fetal heartbeat and contractions, usually once a week until birth. |
When to see a gynaecologist without waiting for your appointment
- vaginal bleeding at any stage of pregnancy, even if light;
- severe or persistent lower abdominal pain, especially one-sided in the first weeks;
- your waters breaking or watery discharge before term;
- absent or noticeably weaker fetal movements after week 28;
- severe headache, visual disturbance or sudden swelling of the face and hands in the second half of pregnancy;
- fever above 38 degrees, persistent vomiting or burning when passing urine.
All of these are reasons for a same-day examination, not for waiting until the next scheduled visit.
Prices of pregnancy check-ups
| Service | Price |
|---|---|
| Gynaecological examination | 10,000 to 12,000 RSD |
| Gynaecological ultrasound | 8,000 RSD |
| Gynaecological examination with ultrasound | 12,000 RSD |
| Expert ultrasound | 10,000 to 12,000 RSD |
| CTG | 7,000 RSD |
| GBS swab | 1,500 RSD |
Frequently asked questions
How do I calculate my due date?
Add 280 days to the first day of your last period, that is seven days and nine months. If your cycle is not 28 days, add the difference as well: with a 32-day cycle the due date is four days later. The calculator at the top of the page does exactly that.
How many weeks pregnant am I?
Weeks are counted from the first day of your last period, not from conception. If 70 days have passed since then, you have completed 10 weeks and are in week 11 of pregnancy. A first-trimester ultrasound may shift that count by a few days.
Why is pregnancy counted from the period and not from conception?
Because most women know the date of their last period, while the date of ovulation and conception is rarely known. By convention pregnancy is counted from that known date, so at the moment of conception the embryo is formally already in week two. That is why fetal age is always about two weeks less than the week of pregnancy.
Can the due date change?
Yes, and it happens often. If the date from a first-trimester ultrasound differs from the period-based date by more than seven days, the gynaecologist corrects the due date to the ultrasound. After that the due date stays fixed, even if later scans show a bigger or smaller baby.
When is the best time for the first visit?
Between weeks 6 and 8, when the heartbeat can already be seen on ultrasound. Earlier than that the scan often cannot yet confirm a normal pregnancy, so it has to be repeated. If you have bleeding or pain, do not wait, come in straight away.
Book an appointment
If you have just found out you are pregnant or one of the check-ups in the table is due, book a gynaecological examination with ultrasound at Miren Medic. Pregnancy care, expert ultrasound, CTG and laboratory tests are all done in one place, without a referral and without waiting.
Related articles
Medically reviewed by
Specialist in gynecology and obstetrics
