Pregnancy Week Calculator
Enter the first day of your last period (with your cycle length), the conception date, or the date of an ultrasound and the gestational age it measured, to see how many weeks and days pregnant you are on a chosen date, which trimester you are in, your estimated due date and how long is left.
- Trimester1st trimester (0 0/7 – 13 6/7 weeks)
- Estimated due date30.05.2027
- Time to the due date210 days (30 weeks, 0 days)
- A birth at this age isPreterm (before 37 weeks)
Dates of key weeks
How it is calculated
Gestational age is counted from the first day of the last menstrual period, not from conception. Under the joint opinion of the American College of Obstetricians and Gynecologists (ACOG), the American Institute of Ultrasound in Medicine and the Society for Maternal–Fetal Medicine (Committee Opinion 700), the estimated due date is by convention 280 days after the first day of the last period. The rule assumes a regular 28-day cycle with ovulation on day 14, so it does not allow for imperfect recall of the period, irregular cycles or variation in when ovulation happens. When a cycle differs from 28 days the difference lies mostly in the phase before ovulation (the follicular phase), so the tool shifts the dates by the difference (a modified Naegele rule). That shift is a convention, not an ACOG recommendation. ACOG considers an ultrasound measurement in the first trimester (before 14 weeks 0 days) the most accurate way to date a pregnancy; when it differs markedly from the last period the doctor redates the pregnancy by the scan, while an ultrasound after 28 weeks is the least reliable method. In IVF pregnancies the date comes from the embryo’s age and the transfer date, which is what the conception option is for. The trimester limits are those in ACOG Committee Opinion 700 (1st trimester before 14 0/7 weeks, 2nd from 14 0/7 to before 28 0/7, 3rd from 28 0/7 weeks). Birth timing classes follow ACOG Committee Opinion 579: early term 37 0/7 to before 39 0/7, full term 39 0/7 to before 41 0/7, late term 41 0/7 to before 42 0/7, postterm from 42 0/7 weeks; a birth before 37 completed weeks is preterm under the World Health Organization’s definition. This page is for information only and is not medical advice.
Start = LMP + (Cycle − 28) or Conception − 14 or Scan date − Gestational age at the scan; Gestational age = Date − Start; EDD = Start + 280 daysLMP: first day of the last menstrual period. Start: the day gestational age is 0 weeks 0 days (the LMP of a 28-day cycle). EDD: estimated due date. Gestational age is found in days and written as completed weeks plus days, e.g. “10 weeks, 2 days”; 10 completed weeks means being in the 11th week of pregnancy.- From the last period: if your cycle is longer than 28 days, move the LMP forward by the difference; if shorter, back (ovulation shifts by the same amount).
- From conception: go back 14 days; conception day is gestational day 14.
- From an ultrasound: subtract the gestational age measured that day (weeks × 7 + days) from the scan date.
- Divide the days from that start to the chosen date by 7: the quotient is completed weeks, the remainder days.
- Add 280 days to the start: the estimated due date (40 0/7 weeks).
- Trimester: 1st before 14 0/7 weeks, 2nd from 14 0/7 to before 28 0/7, 3rd from 28 0/7 weeks.
Example
The last period started on 17.08.2026 and the cycle is 28 days. By 28.10.2026, 72 days have passed: 72 = 10 × 7 + 2, so 10 weeks, 2 days (week 11, 1st trimester). The due date is 17.08.2026 + 280 days = 24.05.2027, 208 days away. With a 32-day cycle the start moves 4 days to 21.08.2026: 9 weeks, 5 days, due on 28.05.2027.
Frequently asked questions
Why are pregnancy weeks counted from the last period?
Most people know the first day of their last period, whereas the day of conception is usually unknown. The convention assumes conception on day 14 of a 28-day cycle, so the pregnancy week is about two weeks more than the time since conception.
The ultrasound week differs from the calculation. Which is right?
Under ACOG, a first-trimester ultrasound measurement is more accurate than the date of the last period, and if they differ markedly the pregnancy is redated by the scan; your doctor makes that decision. If your doctor has redated the pregnancy, use the “Ultrasound” option with that scan’s date and measured age.
When do the trimesters change?
Under ACOG Committee Opinion 700 the 1st trimester lasts until 14 weeks 0 days, the 2nd begins at 14 0/7 weeks and the 3rd at 28 0/7 weeks.
What if my cycle is not 28 days?
The tool shifts the dates by the difference from 28 days: 4 days forward for a 32-day cycle, 3 days back for a 25-day one. If your cycle is outside the 24–38 days FIGO counts as normal, or irregular, the estimate from the last period is weak and your doctor will date the pregnancy by ultrasound.
Will the baby come on the due date?
The due date is day 280 (40 weeks 0 days), not a fixed date. ACOG counts a birth from 39 0/7 to before 41 0/7 weeks as full term, from 41 0/7 to before 42 0/7 as late term and from 42 0/7 weeks as postterm. Talk to your doctor about the timing of the birth and monitoring.
When should I seek care straight away?
The danger signs in Türkiye’s Ministry of Health antenatal care guideline: severe nausea and vomiting, vaginal bleeding, seizures, severe headache, blurred vision, fever, being unable to manage daily activities, abdominal pain, difficult or fast breathing, swelling of the face, hands and legs, fewer baby movements, leaking fluid and unexplained itching. If any of these happens, seek care without delay.
Source
- ACOG, AIUM, SMFM — Committee Opinion No. 700, Methods for Estimating the Due Date, Obstet Gynecol 2017;129(5):e150–e154, doi:10.1097/AOG.0000000000002046 (reaffirmed 2025): Background (EDD = LMP + 280 days; 28-day cycle, ovulation on day 14), “Clinical Considerations in the First/Second/Third Trimester” (trimester limits 13 6/7, 14 0/7–27 6/7, 28 0/7; first-trimester ultrasound most accurate, third-trimester least reliable; ART: day-5 embryo EDD = transfer + 261 days) — archived copy of ACOG’s page
- ACOG, SMFM — Committee Opinion No. 579, Definition of Term Pregnancy, Obstet Gynecol 2013;122(5):1139–40, doi:10.1097/01.AOG.0000437385.88715.4a (reaffirmed 2025), Box 1: early term 37 0/7–38 6/7, full term 39 0/7–40 6/7, late term 41 0/7–41 6/7, postterm 42 0/7 and beyond — archived copy of ACOG’s page
- ACOG Committee Opinion No. 700 — current ACOG page (abstract, “Reaffirmed 2025”)
- World Health Organization, Preterm birth fact sheet (10 May 2023), “Overview”: born alive before 37 completed weeks is preterm
- Darwish NA, Thabet SM, Aboul Nasr AL, El Sharkawy S, El Tamamy MN, Modified Naegele’s rule for determination of the expected date of delivery irrespective of the cycle length, Med J Cairo Univ 1994;62(1):39–47, p. 46 (Discussion): LMP + 7 days + 9 months, plus or minus the days by which the cycle exceeds or falls short of 28; the variation mostly affects the follicular phase (WHO EMRO IMEMR copy)
- Bull JR et al., Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles, NPJ Digit Med 2019;2:83, doi:10.1038/s41746-019-0152-7, “Cycle characteristics by cycle length”: the follicular phase is 11.0 days longer in very long and 5.4 days shorter in very short cycles (real-world evidence that cycle differences lie in the follicular phase)
- Republic of Türkiye Ministry of Health, General Directorate of Public Health, Antenatal Care Management Guideline (Doğum Öncesi Bakım Yönetim Rehberi), publication no. 925, Ankara 2018: item 2.6 (p. 9: LMP, gestational age by ultrasound when unknown, due date) and item 6.2 (p. 16: danger signs in pregnancy) — copy in WHO’s policy database
- Munro MG et al. (FIGO), The FIGO ovulatory disorders classification system, Int J Gynaecol Obstet 2022;159(1):1–20, doi:10.1002/ijgo.14331, section 6.1.1: normal cycle 24–38 days at ages 18–45