Prenatal Calculator
A prenatal calculator turns one date you know into the timeline you will plan around: your estimated due date, how many weeks and days pregnant you are today, and the dates your trimesters change over. This one offers three ways in, from the first day of your last period (adjusted for your cycle length), from a known conception date, or from a due date an ultrasound already gave you.
Prenatal Calculator
Due date, weeks pregnant and trimester dates, from your last period, conception date or ultrasound date.
Estimates only. If an early ultrasound gave you a different due date, the scan date is the one your clinician will use. Only about 1 in 20 babies arrives exactly on the estimated date.
Quick Answer
For a last period starting January 1, 2026 with a 28-day cycle, the estimated due date is October 8, 2026, which is 280 days later. The calculator marks the 12-week point at March 26, 2026 and the 28-week point at July 16, 2026. Enter the same conception date (January 15, 2026) instead and you get the same October 8 date, because the tool adds 266 days (38 weeks) from conception.
Key takeaways
- A due date is an estimate, not an appointment. Only about 5% of babies arrive exactly on theirs, and most arrive within two weeks either side.
- Pregnancy is counted from the first day of your last period, about two weeks before conception actually happens, which is why gestational age looks two weeks ahead of fetal age.
- Longer cycles push the estimate later and shorter cycles pull it earlier, by the number of days your cycle differs from 28.
- An early ultrasound beats every date calculation. ACOG treats a first-trimester scan as the most accurate dating method and re-dates the pregnancy when the scan disagrees with the last-period date by more than a few days.
- Quick Answer
- How to use the prenatal calculator
- The formula: Naegele's rule and the two variants
- Worked example: last period January 1, 2026
- Understanding your trimester dates, and one honest caveat
- What competing calculators miss
- How accurate is this, really?
- Planning the rest of the timeline
- Frequently asked questions
- Add This Calculator to Your Website
How to use the prenatal calculator
Pick the dating method, fill in the date, click Calculate. The results stay hidden until you calculate, so nothing on the page is a guess about your pregnancy.
- Choose your dating method: last menstrual period is the standard choice and is pre-selected. Switch to conception date if you tracked ovulation or had fertility treatment, or to ultrasound due date if a scan already gave you one.
- Enter the date: for the last-period method, use day one of that period, not the day it finished. The example (January 1, 2026) is pre-filled so you can see a full result immediately.
- Set your cycle length: leave 28 days unless you know your average is different. This field only affects the last-period method.
- Read the timeline: the big date is the estimated due date, under it is how far along you are today and your current trimester, and the cards below give the milestone dates to put on a calendar.
Reset restores the worked example (January 1, 2026, 28-day cycle) and hides the results again. Copy results and Share results sit under the timeline for saving or sending the numbers.
The formula: Naegele's rule and the two variants
The last-period method uses Naegele's rule, the same arithmetic obstetricians have used for about two centuries: add 280 days (40 weeks) to the first day of the last menstrual period, then adjust for cycle length.
Conception method: due date = conception date + 266 days
Ultrasound method: LMP = ultrasound due date − 280 days
Why 280 versus 266? Gestational age is counted from the last period, but conception usually happens around ovulation, roughly two weeks later. The 280-day figure includes those two weeks; the 266-day figure starts at conception itself. The ACOG committee opinion on estimating the due date describes both the last-period and ultrasound approaches, and notes that ultrasound measurement of the embryo or fetus in the first trimester is the most accurate method to establish gestational age.
Gestational age today is simply the days between the (possibly back-calculated) last period and today, split into weeks and leftover days. The trimester marker uses two cutoffs: up to 84 days (12 weeks) reads as the first trimester, up to 196 days (28 weeks) as the second, and beyond that as the third. The milestone cards show the calendar dates of those two cutoffs plus the due date itself.
Worked example: last period January 1, 2026
The pre-filled example runs exactly as the tool computes it. Last period January 1, 2026, 28-day cycle:
Change only the cycle length to 35 days and the due date moves a week later, to October 15, 2026, because ovulation in a longer cycle happens later. Use the conception method with January 15, 2026 and the due date lands on the same October 8, 2026, while the tool back-dates the last period to January 1, 2026, keeping every number consistent. These are real runs of the calculator, verified in testing, so you can reproduce each one.
Understanding your trimester dates, and one honest caveat
The calculator marks the first trimester through the 12-week point (March 26, 2026 in the example) and the second through the 28-week point (July 16, 2026). Treat these as the tool's fixed milestone dates, not a medical ruling, because the sources themselves do not agree on the exact boundaries.
| Source | First trimester ends | Second trimester ends |
|---|---|---|
| This calculator | 12 weeks (day 84) | 28 weeks (day 196) |
| Cleveland Clinic | 12 weeks | 27 weeks |
| Nemours KidsHealth | End of week 12 | End of week 26 |
| ACOG (clinical dating) | 13 weeks 6 days | 27 weeks 6 days |
For planning purposes the differences do not matter much: the screening windows your clinician cares about, such as the nuchal translucency scan around 11 to 13 weeks and the anatomy scan around 18 to 20 weeks, are scheduled by gestational age, not by which trimester label a calculator prints. The Cleveland Clinic's week-by-week fetal development guide is a solid companion for what is actually happening at each of these dates.
What competing calculators miss
Due date tools fall into a few camps, and each leaves something out:
- Single-method tools (most template calculator sites): last period in, due date out, no cycle-length adjustment and no conception or ultrasound path. If your cycles run 35 days, their answer is silently a week early.
- Auto-calculating pages (several ad-heavy due date pages): results appear before you finish typing, which makes the worked example impossible to audit and confuses anyone checking the math.
- Thin content-farm tools: a due date with no trimester context, no milestones, and prose that claims "exact" dates while admitting nowhere that an ultrasound re-dates the pregnancy.
- The gap this page fills: three dating methods with the arithmetic shown, cycle-length adjustment that actually moves the date, trimester milestone dates on a calendar, an explicit statement that the ultrasound date wins, and a worked example you can re-run to the exact day.
How accurate is this, really?
Think of the result as accurate to about one to two weeks for most pregnancies when the last period is certain and cycles are regular. The Nemours KidsHealth pregnancy calendar puts it plainly: most babies are born between 38 and 42 weeks from the first day of the last period, and only a small share arrive on the due date itself.
The estimate gets weaker when the inputs are shaky: irregular cycles, recent hormonal contraception, breastfeeding, or an uncertain last-period date all widen the error. That is exactly when ACOG's guidance matters most, because a first-trimester ultrasound dates the pregnancy to within a few days regardless of cycle history. Use this calculator to plan and to follow along week by week, then let the scan set the official date. The NICHD pregnancy overview and MedlinePlus pregnancy guide both stress early, regular prenatal care, which no calculator replaces.
One more number worth knowing: the CDC reports that about 1 in 10 U.S. births is preterm (before 37 weeks), and advises against scheduling delivery before 39 weeks without a medical reason, since the brain, lungs and liver keep developing in those final weeks. The due date is the midpoint of a window, not a deadline.
Planning the rest of the timeline
Once the due date is set, the rest of the planning follows from it. If you conceived through IVF, the IVF conception calculator handles transfer-date dating, which ACOG prefers over last-period math for assisted reproduction. A forward due date calculator is handy for projecting any future date forward by weeks, and the pregnant weight gain calculator tracks the other number your clinician will watch at every visit. For a sense of population context rather than personal timing, the conception rate calculator works through fertility statistics.
Frequently asked questions
For regular 28-day cycles with a certain last-period date, the estimate is usually within one to two weeks of when you will actually give birth. Only about 5% of babies arrive exactly on the estimated date, and most arrive within two weeks either side. An early ultrasound is more precise: ACOG considers a first-trimester scan the most accurate dating method, and your clinician will re-date the pregnancy when the scan and the last-period date disagree by more than a few days.
Because the exact day of conception is hard to pin down in most pregnancies, while the first day of the last period is a date people can usually name. Gestational age therefore starts about two weeks before conception, at ovulation. That is why a pregnancy is described as 40 weeks (280 days) from the last period but only 38 weeks (266 days) from conception. They are the same pregnancy measured from two different starting lines.
The standard rule assumes a 28-day cycle with ovulation around day 14. If your cycles are longer, ovulation happens later, so the due date moves later by the difference: a 35-day cycle pushes the estimate about a week later than a 28-day cycle would. A shorter cycle pulls it earlier by the same logic. This calculator applies that adjustment automatically with the formula due date = LMP + 280 days + (cycle length − 28).
Yes, if you are confident about it, for example from ovulation tracking or fertility treatment. The calculator adds 266 days (38 weeks) to the conception date and back-dates the last period by 14 days so the weeks-pregnant count stays consistent. For IVF pregnancies, ACOG advises using the embryo age and transfer date to set the due date rather than last-period math.
They are calendar dates for two fixed points in the pregnancy: the 12-week mark and the 28-week mark from the last period, plus the due date. They are planning anchors, not medical definitions, because published sources place the trimester boundaries slightly differently: some use 12 weeks, others 13 weeks 6 days, for the end of the first trimester. Your clinician schedules screenings by gestational age, not by the trimester label.
You can get a rough timeline from it, but ACOG's guidance is that pregnancies from assisted reproductive technology should be dated from the embryo age and transfer date, which your clinic will have given you. The conception-date method here approximates that (266 days from conception), and the IVF conception calculator on this site handles transfer-date dating directly. Treat the clinic's date as the official one.
First, do not panic: going past the estimated date is common, especially with a first baby. ACOG defines late term as 41 weeks 0 days through 41 weeks 6 days and postterm as 42 weeks and beyond, and your provider will discuss monitoring and induction options as those points approach. Contact your provider promptly if you notice reduced fetal movement or any warning signs, rather than waiting for the date to resolve itself.
No. It is an educational tool that applies standard obstetric dating arithmetic to the dates you enter. It cannot account for irregular cycles, uncertain dates, multiples, or complications, and it is not a substitute for prenatal care. Confirm the dates with your healthcare provider, preferably with an early ultrasound.
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References
- American College of Obstetricians and Gynecologists. Methods for Estimating the Due Date, Committee Opinion No. 700. Accessed October 23, 2026.
- American College of Obstetricians and Gynecologists. Definition of Term Pregnancy, Committee Opinion No. 579. Accessed October 23, 2026.
- American College of Obstetricians and Gynecologists. How Your Fetus Grows During Pregnancy, FAQ156. Accessed October 23, 2026.
- World Health Organization. WHO antenatal care recommendations: imaging ultrasound before 24 weeks of pregnancy, NCBI Bookshelf. Accessed October 23, 2026.
- World Health Organization. Recommendations on antenatal care interventions along the life course. Accessed October 23, 2026.
- Cleveland Clinic. Fetal Development: Week-by-Week Stages of Pregnancy. Accessed October 23, 2026.
- Centers for Disease Control and Prevention. Preterm Birth. Accessed October 23, 2026.
- Eunice Kennedy Shriver National Institute of Child Health and Human Development. Pregnancy. Accessed October 23, 2026.
- U.S. National Library of Medicine. Pregnancy, MedlinePlus. Accessed October 23, 2026.
- Nemours KidsHealth. A Week-by-Week Pregnancy Calendar. Accessed October 23, 2026.