Pregnancy Due Date Calculator
Calculate your estimated due date from LMP, conception date, ultrasound, or IVF transfer — privately, instantly, in your browser.
What it does
Five calculation methods
Calculate from last menstrual period (LMP), conception date, ultrasound gestational age, IVF embryo transfer date (day 3 or day 5), or a known due date.
Cycle-length adjustment
The LMP method lets you adjust for cycle lengths from 20 to 45 days, correcting the standard Naegele's rule for cycles that differ from the 28-day average.
Full result panel
See your estimated due date, current week and day of pregnancy, trimester, days remaining, and a visual progress bar showing how far along you are.
Milestone timeline
A dated timeline of key events: first prenatal visit, end of first trimester, anatomy scan window, viability at 24 weeks, third trimester, full term, due date, and post-term threshold.
Week-by-week guide
A complete 40-week pregnancy guide with baby size comparisons, fetal development highlights, and what you may experience each week — sourced from ACOG, NHS, and Mayo Clinic guidance.
Calendar export and sharing
Download a .ics calendar file to add your due date and milestones to any calendar app. Copy your result to clipboard or share via the native Web Share API on mobile.
Completely private
All calculations run in JavaScript in your browser. Your dates are never transmitted to any server, stored in any database, or shared with any third party.
How to use Pregnancy Due Date Calculator
- 1Choose your calculation method
Select from five tabs: Last Menstrual Period (LMP), Conception Date, Ultrasound, IVF Transfer Date, or Known Due Date. If you're unsure, LMP is the most common starting point.
- 2Enter your dates
Fill in the required dates for your chosen method. For LMP, you can also adjust your cycle length using the slider if your cycle isn't exactly 28 days.
- 3Tap Calculate
Press the Calculate button to run the estimation. Results appear immediately below with your due date, gestational age, trimester, and key milestones.
- 4Review your results
See your estimated due date prominently displayed, along with your current week of pregnancy, trimester badge, days remaining, and a full milestone timeline.
- 5Save or share
Copy your result to clipboard, download a .ics file to add milestones to your calendar app, or use the Share button to send the information to a partner or family member.
When to use this
Just found out you're pregnant
Enter the first day of your last period and your typical cycle length to get your estimated due date, how far along you are, and what trimester you're in — all in one tap.
Tracking week-by-week progress
Use the week-by-week guide to follow fetal development and understand what's happening each week of your pregnancy, including what you might be experiencing.
IVF or assisted reproduction
Use the IVF transfer date method to calculate your due date from a day-3 or day-5 embryo transfer — the formulas differ from the standard LMP method.
Common errors & fixes
- "This date is in the future" when entering LMP
- The last menstrual period must be a past date. If your period started recently, enter today's date or the actual start date of your last period.
- "This date seems too far in the past" for LMP
- If the LMP was more than 44 weeks ago, the pregnancy would be past the expected post-term window. Double-check the date — you may have entered the wrong year or month.
- Due date appears to be already in the past
- If your calculated due date is in the past, you may have entered an LMP from a previous pregnancy or an incorrect date. Re-enter the first day of the most recent period before this pregnancy.
- IVF result seems off
- Make sure you've selected the correct transfer day (Day 3 or Day 5). Day 5 blastocyst transfers are more common in modern IVF and use a different calculation (261 days) compared to Day 3 (263 days).
Technical details
| LMP method formula | EDD = LMP + 280 days + (cycle length − 28) days |
| Conception method formula | EDD = conception date + 266 days |
| Ultrasound method formula | EDD = ultrasound date + (280 − gestational age in days) |
| IVF day-5 formula | EDD = transfer date + 261 days |
| IVF day-3 formula | EDD = transfer date + 263 days |
| Trimester boundaries | T1: weeks 1–13; T2: weeks 14–27; T3: weeks 28–40+ |
| Full-term definition | 37 weeks 0 days to 40 weeks 6 days (ACOG) |
| Date arithmetic | All calculations use local-date strings (YYYY-MM-DD) with integer day offsets — no UTC conversion, no timezone bugs |
| Privacy | 100% client-side JavaScript — no dates are transmitted to any server |
How healthcare providers calculate due dates
The standard method for estimating a due date is Naegele's rule, named after the German obstetrician Franz Karl Naegele, who described it in the early 19th century. The rule adds 280 days — or 40 weeks — to the first day of the last menstrual period, then adjusts for cycle length variation.
This approach treats the LMP as the start of gestational age, even though conception doesn't actually occur until about two weeks later. That's why gestational age is always about two weeks ahead of "fertilization age" or "embryonic age." A pregnancy at 8 weeks gestational age is only 6 weeks from conception.
First-trimester ultrasound dating is widely regarded as more accurate than LMP-based calculations, particularly when the LMP date is uncertain or the cycle is irregular. The American College of Obstetricians and Gynecologists (ACOG) recommends that if a first-trimester ultrasound differs from the LMP-based date by more than 7 days, the ultrasound should be used to establish the EDD. After the first trimester, growth variability makes ultrasound less reliable for changing an established due date.
Why due dates are estimates, not deadlines
A common source of anxiety for expectant parents is the framing of the "due date" as a precise deadline. In reality, only about 4–5% of babies arrive on their estimated due date. The vast majority are born within a window of plus or minus two weeks. Statistically, first-time parents tend to go into labor slightly later than the EDD, while those who have given birth before tend to go slightly earlier.
The term "estimated due date" (EDD) is more accurate than "due date" — it's the midpoint of a probability distribution, not a firm delivery date. Full-term pregnancy is defined by ACOG as 39 weeks 0 days to 40 weeks 6 days. Births between 37 and 38 weeks are "early term," 41 weeks is "late term," and 42 weeks and beyond is "post-term."
This matters because there can be real differences in outcomes between early-term and full-term births, particularly for fetal brain development, lung maturity, and breastfeeding success. The shift away from routine early elective inductions before 39 weeks in recent years reflects a growing recognition that every additional week in the womb during this period has developmental benefits.
IVF and assisted reproduction due date calculations
For pregnancies achieved through in vitro fertilization (IVF), the calculation method differs from the standard LMP approach because the exact date and developmental stage of the embryo at transfer are known precisely.
In a day-5 transfer (blastocyst stage), the embryo is already 5 days old at transfer, so 261 days are added to the transfer date. In a day-3 transfer, 263 days are added. These numbers account for the 2-day difference in embryonic age at the time of uterine transfer, while still targeting the same 40-week gestational endpoint.
For frozen embryo transfers (FET), the same formulas apply. In donor egg cycles, some providers count gestational age from the retrieval date of the egg, while others use the transfer date — it's worth confirming with your reproductive endocrinologist which convention they use. IVF pregnancies can sometimes be dated more precisely than natural conception pregnancies, because the transfer date and embryo age are both known with certainty.
Frequently Asked Questions
How is a due date calculated?
- The most common method is Naegele's rule, which adds 280 days (40 weeks) to the first day of your last menstrual period. This assumes a 28-day cycle with ovulation on day 14. For longer or shorter cycles, the calculation is adjusted accordingly. Ultrasound measurements can also estimate or confirm a due date based on fetal size.
How accurate is a due date calculator?
- An estimated due date (EDD) is just that — an estimate. Research shows that only about 4–5% of babies are born on their exact due date. The vast majority arrive within two weeks either side. Ultrasound dating is generally more accurate than LMP-based calculations, especially when performed in the first trimester. Your healthcare provider may adjust your due date based on ultrasound measurements.
What is the most common way to estimate a due date?
- The most widely used method is the Last Menstrual Period (LMP) method, which adds 280 days to the first day of the last period. This is standard practice because most people know this date even if they're unsure of the exact conception date. First-trimester ultrasound is considered the most accurate method for dating a pregnancy.
Can my due date change?
- Yes, due dates can and do change. If an early ultrasound shows the baby's size is significantly different from what the LMP date would suggest, your provider may adjust the EDD. Dates are most accurately established in the first trimester before growth variability increases. Subsequent ultrasounds are less reliable for changing a due date that's already been established.
How many weeks is a full-term pregnancy?
- A full-term pregnancy is defined as 39 weeks 0 days to 40 weeks 6 days by the American College of Obstetricians and Gynecologists (ACOG). "Early term" is 37–38 weeks, "late term" is 41 weeks, and "post-term" is 42 weeks and beyond. Babies born before 37 weeks are considered preterm.
What does LMP mean?
- LMP stands for Last Menstrual Period — the first day of the last menstrual cycle before pregnancy. Even though conception typically occurs about two weeks after the LMP, gestational age is counted from this date because it's a more reliably known date than the conception date. This means pregnancy is technically "two weeks old" at conception.
How is an IVF due date calculated differently?
- IVF due dates are calculated from the embryo transfer date rather than LMP, because the timing of fertilization is precisely known. For a day-5 (blastocyst) transfer, 261 days are added to the transfer date. For a day-3 transfer, 263 days are added. These formulas account for the embryo's stage of development at transfer.
Can I calculate my due date from an ultrasound?
- Yes. If you know the date of your ultrasound and the gestational age measured at that scan (e.g., "8 weeks and 3 days"), you can calculate the due date by working backward. Subtract the gestational age from 280 days and add the difference to the ultrasound date. Our ultrasound method tab does this automatically.
What if I have irregular periods?
- Irregular periods make LMP-based calculations less reliable because the assumption of a 28-day cycle with ovulation on day 14 may not apply. If your cycles are consistently longer or shorter, adjusting the cycle length slider in the LMP method will improve accuracy. For very irregular cycles, an early first-trimester ultrasound is the most reliable way to establish a due date.
What percentage of babies are born on their due date?
- Only about 4–5% of babies are born on their estimated due date. Most births occur in a window of two to three weeks around the due date. From 37 to 42 weeks is considered the normal birth window. If you reach 42 weeks without going into labor, your provider will typically recommend monitoring and may suggest induction.
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