Use this pregnancy by week calculator to estimate your gestational age, current pregnancy week, trimester and estimated due date. Calculate using your last menstrual period, due date, conception date, ultrasound information or IVF transfer date — whichever you know.
Pregnancy by Week Calculator
Calculator
Your cycle length can shift the estimate because ovulation doesn't happen on exactly the same day for everyone. Adjusting this does not make the result clinically definitive.
We'll calculate backward from this date. Your clinician may establish a different due date based on clinical information such as an ultrasound.
Pregnancy is usually dated from the first day of the last menstrual period, not from conception. Because the exact date of conception can be hard to know, this method gives an estimate, assuming conception occurred about two weeks after the start of a cycle.
Ultrasound dating may be used by healthcare professionals when menstrual dating is uncertain, or when there's a meaningful difference between LMP-based and ultrasound-based dating. This calculator does not tell you which date to use clinically.
This uses the standard IVF dating convention. Your fertility or obstetric care team should confirm your dating.
Your dates are calculated in your browser and are not sent to our server.
Your Week-by-Week Guide
How Many Weeks Pregnant Am I?
Pregnancy is typically counted in completed weeks and days from the first day of your last menstrual period (LMP) — this is called gestational age. Because ovulation usually happens roughly two weeks into a cycle, this convention means the first two weeks of a pregnancy timeline are counted before conception actually occurs. If you don't know your LMP, you can estimate your pregnancy week using your due date, an ultrasound, a known conception date, or an IVF transfer date instead, using the calculator above.
How This Pregnancy Calculator Works
Choose the dating method that matches what you know, enter the requested date (and, for ultrasound or IVF, a couple of extra details), and the calculator estimates your gestational age, trimester, estimated due date, estimated conception window and pregnancy progress. All the calculations run locally in your browser using standard, widely used pregnancy-dating conventions — no information is sent anywhere.
Calculate Pregnancy Weeks by LMP
The most common method estimates your due date as 280 days (40 weeks) after the first day of your last period, following Naegele's rule. If your average cycle is longer or shorter than 28 days, the calculator can adjust the estimate, since a longer or shorter cycle generally shifts the likely date of ovulation.
Calculate Pregnancy Weeks by Due Date
If you already have an estimated due date from a provider, the calculator works backward, assuming a standard 280-day pregnancy, to estimate your current gestational age, likely LMP and approximate conception window.
Calculate Pregnancy Weeks by Conception Date
If you know or estimate when conception occurred — for example, from ovulation tracking — the calculator adds roughly two weeks to align with the standard LMP-based dating convention, since clinical dating is usually expressed from the last period rather than from conception itself.
Calculate Pregnancy Weeks by Ultrasound
Early ultrasounds can estimate gestational age based on measurements of the developing pregnancy. Enter the ultrasound date and the gestational age shown on the report, and the calculator projects that dating forward to today. Providers sometimes use ultrasound dating instead of LMP-based dating, particularly when LMP is uncertain or the two estimates differ meaningfully.
Calculate Pregnancy Weeks After IVF
For pregnancies conceived through IVF, dating is typically based on the embryo transfer date and the embryo's age at transfer (day 3, day 5, or day 6), since the fertilization date is known precisely. This is generally considered a more precise starting point than LMP-based dating.
Pregnancy Weeks to Months
Pregnancy is normally measured in weeks and days rather than months, because calendar months aren't all the same length and "month" boundaries aren't standardized in medical practice. The table below gives a general, approximate mapping for reference.
| Pregnancy weeks | Approximate month |
|---|---|
| Weeks 1–4 | Month 1 |
| Weeks 5–8 | Month 2 |
| Weeks 9–13 | Month 3 |
| Weeks 14–17 | Month 4 |
| Weeks 18–22 | Month 5 |
| Weeks 23–27 | Month 6 |
| Weeks 28–31 | Month 7 |
| Weeks 32–35 | Month 8 |
| Weeks 36–40+ | Month 9 |
Pregnancy Trimesters by Week
This calculator uses one consistent trimester methodology throughout: the first trimester spans weeks 1–13, the second trimester spans weeks 14–27, and the third trimester spans weeks 28 through delivery. Some sources use slightly different boundaries, so your provider's chart may vary by a few days.
40-Week Pregnancy Timeline
Use the week-by-week guide inside the calculator above to browse general, educational information for any week from 1 through 40 — including typical development notes, common symptoms, and what a prenatal visit around that week might involve.
Why Your LMP and Ultrasound Dates May Differ
It's common for LMP-based dating and ultrasound-based dating to differ by several days, especially when cycles are irregular, the exact start date of the last period is uncertain, or ovulation happened earlier or later than the average assumption of day 14. Early ultrasounds are generally considered a reliable way to refine dating, and healthcare providers may use ultrasound findings together with clinical judgment when establishing your official pregnancy dating. If your dates differ, that's a normal part of estimating — it isn't something to adjust yourself, and it's worth discussing with your provider.
How Accurate Is a Pregnancy Week Calculator?
Pregnancy calculators like this one provide estimates, not clinical determinations. LMP-based calculations depend on accurately remembering the first day of your last period and assume a textbook cycle; cycle length and the timing of ovulation vary between people and even cycle to cycle. Ultrasound-based dating, particularly in the first trimester, is often considered more precise for estimating gestational age, but it is still an estimate. Your healthcare provider is the right source for your official pregnancy dating.
What If I Don't Know My LMP?
If you're unsure of the first day of your last period, you can still get a useful estimate using any of the other information you have: an estimated due date already given to you by a provider, an ultrasound gestational-age reading, an estimated conception date, or an IVF embryo transfer date. If none of that information is available or your pregnancy can't be reliably dated, a healthcare provider can help establish gestational age through an exam or ultrasound.
Frequently Asked Questions
How many weeks pregnant am I?
Enter the information you know — your last period, due date, conception date, ultrasound reading, or IVF transfer date — into the calculator above to get an estimate of your current pregnancy week and day.
How is pregnancy week calculated?
Pregnancy week is conventionally calculated as the number of completed weeks and days since the first day of your last menstrual period, even though conception generally occurs about two weeks after that date.
How many weeks is a full-term pregnancy?
A full-term pregnancy is generally considered to be around 39 to 40 weeks, though a pregnancy that reaches 37 weeks is typically described as early term. Actual delivery timing varies.
Does pregnancy start at conception or LMP?
Biologically, pregnancy begins at conception. However, gestational age is conventionally dated from the first day of the last menstrual period for consistency, since the LMP date is usually easier to identify than the exact date of conception.
Why am I two weeks pregnant when I just conceived?
Because gestational age counts from the first day of your last period rather than from conception, and ovulation typically happens roughly two weeks into a cycle, it's normal for the dating convention to show about two weeks of pregnancy at the time of conception.
Can I calculate pregnancy weeks from my due date?
Yes. Select "I know my due date" in the calculator above, and it will estimate your current gestational age by counting back from a standard 280-day pregnancy length.
Can I calculate pregnancy weeks without knowing my LMP?
Yes. You can use your estimated due date, an ultrasound reading, an estimated conception date, or an IVF transfer date instead — the calculator above supports all of these methods.
Can an ultrasound change my pregnancy due date?
A healthcare provider may adjust your estimated due date based on ultrasound measurements, particularly if there's a meaningful difference from LMP-based dating. This is a clinical decision made by your provider, not something this calculator determines for you.
How accurate is a pregnancy week calculator?
These calculators provide an estimate based on standard dating conventions and the information you enter. They can't account for individual variation in cycles or ovulation timing the way a clinical exam or ultrasound can, so treat the result as a helpful estimate rather than a precise clinical determination.
How many months is 20 weeks pregnant?
20 weeks falls at approximately month 5 using the general weeks-to-months mapping in the table above, and marks roughly the midpoint of a standard pregnancy timeline.
How do I calculate pregnancy weeks after IVF?
Select "I had IVF" in the calculator above and enter your embryo transfer date and the embryo's age at transfer (day 3, 5, or 6). Because the fertilization date is known, IVF dating is generally considered precise, but your fertility team should confirm it.
What if my periods are irregular?
If your cycles are irregular, LMP-based dating may be less reliable. You may want to use the due-date, ultrasound, or conception-date methods instead, and discuss dating with your healthcare provider, who may recommend an ultrasound to help establish gestational age.
What if my calculated pregnancy week doesn't match my ultrasound?
Small differences between LMP-based and ultrasound-based estimates are common and generally expected. Your healthcare provider will typically use clinical judgment, and sometimes ultrasound findings, to determine your official pregnancy dating — this isn't something to resolve on your own.
Medical References
This tool is for general education and does not replace medical advice. For authoritative information on pregnancy dating and prenatal care, see resources such as the American College of Obstetricians and Gynecologists (ACOG), MedlinePlus, the CDC, the NIH, and the NHS.