Pregnancy Due Date
Calculator
Calculate your estimated due date using 4 different methods. Track your baby's growth, explore week-by-week development, and prepare for your journey ahead ā all in one place.
Choose Method
Last Menstrual Period
Most common method (Naegele's Rule)
Conception Date
If you know when you conceived
IVF Transfer Date
For IVF or frozen embryo transfer
Ultrasound Estimate
From your doctor's ultrasound scan
Default 28 days. Adjust if your cycle is different.
Select a method and enter your date to see your results
Your Estimated Due Date
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Pregnancy Progress
0%How Far Along
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Trimester
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Days Remaining
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Conception Est.
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Your Baby This Week
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Trimester Breakdown
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Fetal Growth Curve
Birth Probability Timeline
Based on statistical distribution ā only ~5% of babies arrive on their due date.
Key Dates & Milestones
Select any week to see your baby's development:
Click a week number above to see details
Pregnancy Due Date Calculation ā ToolRiz.com
What Is a Pregnancy Due Date Calculator?
A pregnancy due date calculator is a tool that estimates the date you are most likely to give birth based on specific information about your pregnancy. The most widely used method is based on the first day of your last menstrual period (LMP), using a formula called Naegele's Rule that has been standard in obstetric care since the 1800s. This calculator adds 280 days (40 weeks) to your LMP to arrive at an estimated delivery date, or EDD.
According to the American College of Obstetricians and Gynecologists (ACOG), a full-term pregnancy is defined as 39 weeks to 40 weeks and 6 days. However, only about 4ā5% of babies are actually born on their exact due date. Most babies arrive within a two-week window before or after the calculated date. This is why healthcare providers prefer the term "estimated due date" rather than "exact due date."
This calculator goes beyond basic LMP calculation by offering four different methods: last menstrual period, known conception date, IVF transfer date, and ultrasound-based estimation. It also provides your current gestational age, trimester, fetal size comparisons, a growth curve chart, birth probability timeline, and key milestone dates ā making it one of the most comprehensive free pregnancy calculators available to US expectant mothers.
How Is a Due Date Calculated?
The standard method for calculating a due date is Naegele's Rule, developed by German obstetrician Franz Naegele in 1830. The formula is straightforward:
This formula assumes a 28-day menstrual cycle with ovulation occurring on day 14. If your cycle is longer or shorter than 28 days, the calculator adjusts accordingly. For example, if you have a 35-day cycle, ovulation likely occurs around day 21, so the due date would be pushed back by approximately 7 days.
Here's how the 280-day (40-week) timeline breaks down:
An important distinction: gestational age (calculated from LMP) is typically about 2 weeks more than fetal age (calculated from conception). When your doctor says "you're 8 weeks pregnant," they mean 8 weeks gestational age, which means the fetus is actually about 6 weeks old.
The 4 Due Date Calculation Methods
1. Last Menstrual Period (LMP)
The most universal method. Adds 280 days to the first day of your last period. Works best with regular 28-day cycles. ACOG recommends this as the standard method when the LMP is certain and cycles are regular. If your cycle length differs from 28 days, our calculator adjusts the estimate by adding or subtracting the difference from day 14.
2. Conception Date
If you know the exact date of conception (e.g., from ovulation tracking, fertility treatments, or a single sexual encounter), this method adds 266 days (38 weeks) to that date. This can be more accurate than LMP for women with irregular cycles. However, pinpointing the exact conception date is difficult for most women, as sperm can survive up to 5 days and ovulation timing varies.
3. IVF Transfer Date
For women who conceived through in vitro fertilization, the transfer date is known precisely. For a 5-day blastocyst transfer, the due date is calculated by subtracting 5 days from the transfer date (to get the "conception" equivalent) and then adding 266 days. For a 3-day embryo transfer, subtract 3 days then add 266 days. This is often the most accurate method because the timing is precisely known.
4. Ultrasound Estimation
During early pregnancy (especially weeks 8ā14), an ultrasound measurement of the crown-rump length (CRL) can estimate gestational age within 5ā7 days of accuracy. You enter the ultrasound date and the weeks/days your provider estimated, and the calculator works backward to find the due date. Note: ultrasounds become less accurate for dating as pregnancy progresses, because babies grow at different rates in later trimesters.
Understanding the Three Trimesters
Pregnancy is divided into three trimesters, each marked by distinct developmental milestones and maternal changes. Understanding what happens in each trimester helps you prepare physically and emotionally:
First Trimester (Weeks 1ā13)
The most critical period for fetal development. The neural tube forms (weeks 3ā4), the heart begins beating (week 5ā6), and all major organs begin developing. By week 12, your baby has all its organs, fingers, toes, and even fingerprints. For the mother, this trimester often brings morning sickness (nausea), fatigue, breast tenderness, and frequent urination. The risk of miscarriage drops significantly after week 12 (from ~10% to ~2%). Key medical appointments: first prenatal visit (weeks 8ā10), nuchal translucency ultrasound (weeks 11ā13), and first trimester screening.
Second Trimester (Weeks 14ā27)
Often called the "honeymoon phase" of pregnancy. Morning sickness typically subsides, energy returns, and you'll likely feel your baby move for the first time (quickening, usually between weeks 16ā22). The baby grows rapidly ā from about 3 ounces at week 14 to over 2 pounds by week 27. Major milestones include the anatomy scan ultrasound (weeks 18ā22), which checks the baby's organs, limbs, and placenta. Around week 24, the baby reaches viability ā meaning it could potentially survive outside the womb with medical intervention. Gestational diabetes screening (weeks 24ā28) is another key test this trimester.
Third Trimester (Weeks 28ā40+)
The final stretch. The baby gains most of its weight during this period, adding about half a pound per week in the last months. The lungs mature, the brain develops rapidly, and the baby positions head-down for delivery. For the mother, this trimester brings increased discomfort: back pain, swelling, shortness of breath, Braxton Hicks contractions, and insomnia. Key appointments include weekly or biweekly checkups starting at week 36, Group B Strep testing (weeks 35ā37), and cervical exams. Labor typically begins between weeks 37 and 42.
How Accurate Are Due Date Calculators?
Due date calculators provide an estimate, not a guarantee. Here's what the data shows about their accuracy:
Accuracy depends heavily on the method and individual factors:
- LMP method: Most accurate for women with regular 28-day cycles who are certain of their last period date. Accuracy drops with irregular cycles.
- Conception date: Can be more accurate if the date is truly known, but most women cannot pinpoint it exactly.
- IVF transfer: Typically the most accurate because the date is precisely documented.
- Early ultrasound (weeks 8ā14): Very accurate (Ā±5 days), often used to confirm or adjust the LMP-based date.
- Late ultrasound (after 20 weeks): Less accurate for dating (Ā±2ā3 weeks) because fetal growth rates vary more in later pregnancy.
According to a 2020 study published in PLoS One analyzing over 18 million US births, the median gestational age at delivery was 39 weeks and 1 day, with only 4.4% of births occurring exactly at 40 weeks. First-time mothers tend to deliver slightly later (average 39 weeks 5 days) compared to second-time mothers (average 39 weeks 2 days).
How Healthcare Providers Calculate Your Due Date
In the United States, obstetricians and midwives follow guidelines established by ACOG for determining due dates. The process typically works like this:
At your first prenatal visit (usually weeks 8ā10), your provider will ask for the first day of your last menstrual period and calculate a preliminary due date using Naegele's Rule.
An early ultrasound (ideally between weeks 8ā13 weeks 6 days) is performed. The crown-rump length (CRL) measurement is compared to standardized growth charts to estimate gestational age.
If the ultrasound estimate differs from the LMP estimate by more than 5ā7 days (in the first trimester), ACOG recommends using the ultrasound date instead. In the second trimester, the threshold is 7ā10 days.
The due date is then "locked in" and typically not changed again, even if later ultrasounds suggest a different date. This is because fetal growth variation increases as pregnancy progresses.
It's worth noting that due dates are not changed for IVF pregnancies ā the transfer date calculation is considered definitive. If you've had IVF, your due date is essentially set from day one.
Preparing for Your Delivery
Knowing your estimated due date helps you plan ahead. Here's a timeline of what to prepare and when, based on guidance from the American Pregnancy Association and ACOG:
By Week 20
- Start prenatal classes
- Begin baby registry
- Research pediatricians
- Tour hospital/birth center
By Week 28
- Finalize birth plan
- Pack hospital bag
- Install car seat
- Set up baby's sleep space
By Week 32
- Stock up on baby essentials
- Pre-register at hospital
- Arrange help for postpartum
- Complete maternity leave paperwork
By Week 36
- Wash baby clothes/sheets
- Prep meals for postpartum
- Confirm emergency contacts
- Know when to call your provider
Frequently Asked Questions
ACOG defines full-term as 39 weeks 0 days to 40 weeks 6 days. Early term is 37ā38 weeks, late term is 41 weeks, and post-term is 42+ weeks. While 40 weeks is the standard estimate, the average actual delivery in the US is around 39 weeks 1 day for all pregnancies.
Yes, but typically only once, and only in early pregnancy. If your first-trimester ultrasound shows a gestational age that differs from your LMP-based date by more than 5ā7 days, your provider will adjust the due date. After the first trimester, due dates are rarely changed because fetal growth becomes more variable.
Statistically, yes ā but only by a few days. According to a large 2013 study in Obstetrics & Gynecology, first-time mothers delivered an average of 3ā5 days later than women who had given birth before. However, this is an average ā many first babies arrive early, and the difference is not large enough to change your due date.
Only about 4ā5% of babies are born on their exact due date. Approximately 80% arrive within two weeks (before or after), and about 90% within three weeks. Think of your due date as the center of a two-week window, not a precise deadline.
The LMP method is less accurate for irregular cycles. Our calculator adjusts for cycle length, but if your periods are very irregular (e.g., varying by more than a week), the conception date method (if known) or an ultrasound-based estimate from your healthcare provider will be more reliable. Early ultrasound is the gold standard for dating when cycles are irregular.
The due date calculation is the same for twins ā it's still based on 40 weeks from the LMP. However, twins are more likely to be born early. The average delivery for twins is around 35ā37 weeks. About 60% of twins are born before 37 weeks. Your provider will monitor you more closely and may recommend delivery earlier.
Yes ā select the "IVF Transfer Date" method. This is actually the most accurate method because the transfer date is precisely known. Select whether it was a 3-day embryo or 5-day blastocyst transfer, and the calculator will determine the due date accordingly.
Gestational age is counted from the first day of your last period (typically 2 weeks before conception). Fetal age is counted from the actual date of conception. So at "8 weeks pregnant" (gestational), the fetus is about 6 weeks old. Healthcare providers always use gestational age.