Pregnancy Due Date Calculator — LMP, Conception, or IVF
Calculate your estimated due date using your last menstrual period, conception date, or IVF transfer date. Get your trimester timeline and key pregnancy milestones in seconds.
How to Use This Due Date Calculator
Select your calculation method and enter one date. The calculator does the rest — no account, no email, and no personal data stored.
Method 1 — Calculate from Last Menstrual Period (LMP)
This is the most common method and the one your midwife or doctor will use first. Enter the first day of your last period and your average cycle length. The calculator applies Naegele's Rule, adjusted for cycles other than 28 days, to estimate your due date.
Method 2 — Calculate from Conception Date
If you know roughly when you conceived (for example, from ovulation tracking or a single known date of unprotected sex), enter that date. The EDD is 266 days (38 weeks) from conception.
Method 3 — Calculate from IVF Transfer Date
For IVF pregnancies, the date of embryo transfer is precisely known. Select the 5-day or 3-day option based on the age of your embryo at the time of transfer. A 5-day blastocyst transfer adds 261 days to get the EDD; a 3-day embryo adds 263 days.
What the Results Mean
Your result shows your estimated due date, current gestational age (if the date entered is in the past), trimester, and a table of key pregnancy milestone dates calculated from your EDD.
Understanding Your Due Date Results
Your Estimated Due Date (EDD)
The EDD is the date at which your pregnancy reaches 40 weeks of gestational age (counted from the LMP). It's an estimate — research consistently shows that only about 4-5% of babies are born on their exact due date, with most births occurring within two weeks on either side.
Current Week of Pregnancy
Gestational age is stated in weeks and days — "you are 14 weeks and 3 days pregnant" means 14 full weeks plus 3 additional days have passed since your LMP.
Trimester Breakdown
| Trimester | Gestational Weeks | Key Events |
|---|---|---|
| First | Weeks 1–13 | Embryo/fetal development, morning sickness, nuchal scan (weeks 11-13) |
| Second | Weeks 14–26 | Anatomy scan (~week 20), fetal movement, glucose tolerance test (~week 24-28) |
| Third | Weeks 27–40 | Rapid growth, preparation for birth, GBS screening (~week 35-37), full term (week 39) |
How Pregnancy Due Dates Are Calculated
What Is Naegele's Rule?
Naegele's Rule is the formula underlying most due date calculations. It adds 280 days (40 weeks) to the first day of the last menstrual period. This calculator also adjusts for cycle length differences: if your cycle is 35 days rather than 28, the EDD is shifted 7 days later, because ovulation (and therefore conception) likely occurred later in your cycle.
Conception method: EDD = Conception date + 266 days (38 weeks)
IVF 5-day transfer: EDD = Transfer date + 261 days
IVF 3-day transfer: EDD = Transfer date + 263 days
Why Pregnancies Are Counted from the Last Period, Not Conception
The precise moment of conception is rarely known, but the start of the last period is usually well-remembered. Since ovulation typically occurs 14 days into a 28-day cycle, counting from the LMP produces a reliable starting point that adds approximately 2 extra weeks to the biological pregnancy length — this is why we say "40 weeks pregnant" when the embryo is actually about 38 weeks old.
The Role of Ultrasound in Confirming a Due Date
An early ultrasound (before 14 weeks) measures the embryo's crown-rump length, which is a highly reliable indicator of gestational age at this stage. If the ultrasound date differs by more than 7 days from the LMP-based EDD, your healthcare provider will typically revise the due date to match the ultrasound finding. This is why early prenatal appointments are recommended.
IVF Due Date: 3-Day vs. 5-Day Transfer
In a 5-day (blastocyst) transfer, the embryo is 5 days old at transfer, so its age is already known. Adding 261 days accounts for the remaining 37 weeks and 2 days to reach 40 weeks of gestational age. A 3-day embryo is slightly younger, so 263 days are added instead.
Understanding Pregnancy Trimesters
First Trimester (Weeks 1–13)
This is when the embryo forms all of its major organ systems. Many people experience morning sickness (nausea and vomiting, which can happen at any time of day) and fatigue during this period. The risk of miscarriage is highest in the first trimester and drops significantly after the end of week 12. The nuchal translucency scan (weeks 11-13) screens for chromosomal conditions.
Second Trimester (Weeks 14–26)
Often described as the most comfortable trimester, as early pregnancy nausea typically improves. Most people begin to feel fetal movement for the first time (quickening) between weeks 16-22. The anatomy scan at around week 20 checks fetal development in detail. Blood glucose screening for gestational diabetes usually happens around weeks 24-28.
Third Trimester (Weeks 27–40)
The baby grows rapidly, and you may experience Braxton Hicks contractions, back pain, and disrupted sleep as the pregnancy progresses. Group B streptococcus (GBS) screening happens around weeks 35-37. "Full term" by ACOG definition begins at 39 weeks — babies born before this are considered "early term" even if technically past the 37-week "preterm" threshold.
What Is a "Full-Term" Pregnancy?
ACOG Definitions
The American College of Obstetricians and Gynecologists (ACOG) uses precise terminology to describe gestational age at delivery. Early term is 37 weeks 0 days to 38 weeks 6 days. Full term is 39 weeks 0 days to 40 weeks 6 days. Late term is 41 weeks 0 days to 41 weeks 6 days. Post-term is 42 weeks and beyond. These distinctions matter because outcomes differ significantly even within what was previously called "full term."
Gestational Age vs. Fetal Age
Gestational age is measured from the LMP and is what clinicians use clinically. Fetal age (also called embryonic age or conceptional age) is measured from fertilisation and is approximately 2 weeks less than gestational age. When you read "the embryo is 8 weeks old," that typically refers to gestational age — the embryo itself is biologically closer to 6 weeks post-fertilisation.
Factors That Can Affect Your Due Date Accuracy
Irregular Menstrual Cycles
If your periods are irregular, the LMP method is less reliable because ovulation timing is unpredictable. In this case, an early ultrasound is especially important for getting an accurate EDD.
Unknown Conception Date
If you're unsure of your LMP and don't know the conception date, an early ultrasound becomes the primary tool for dating your pregnancy.
Multiple Pregnancies (Twins and Triplets)
Twin and triplet pregnancies are often born earlier than the standard 40-week EDD — the average delivery for dichorionic twins is around 37-38 weeks. Your care team will discuss the appropriate delivery window for your specific situation.
Ultrasound Redating
If an early ultrasound suggests your baby is measuring ahead of or behind the LMP-based dates by more than a week, your healthcare provider will likely revise the EDD. This is routine and doesn't indicate a problem with your baby's development.
What to Do After Getting Your Due Date
First Prenatal Appointment Timeline
Schedule your first prenatal visit as soon as you know you are pregnant, ideally by week 8. Your provider will confirm the EDD, check your health history, and discuss recommended screening tests and supplements (including folic acid and vitamin D).
Prenatal Tests by Trimester
| Timing | Common Tests |
|---|---|
| Weeks 8-12 | Blood type, Rh factor, CBC, STI screens, urine culture |
| Weeks 11-13 | Nuchal translucency ultrasound, combined first-trimester screening |
| Weeks 15-20 | Cell-free DNA / NIPT (optional), quad screen |
| ~Week 20 | Anatomy ultrasound |
| Weeks 24-28 | Glucose tolerance test (gestational diabetes screening) |
| Weeks 35-37 | Group B streptococcus (GBS) swab |
Tests vary by country, healthcare system, and individual clinical situation. Your healthcare provider will advise which tests are appropriate for you.
Who Should Use This Calculator?
Anyone who has recently found out they are pregnant or believes they may be pregnant, people tracking their cycle who want to estimate a due date if they conceived, and IVF patients who want to calculate their estimated EDD from a transfer date.
Frequently Asked Questions
How accurate is a due date calculator?
Accurate to within 1-2 weeks for most pregnancies with a regular cycle and reliable LMP date. Only about 5% of babies arrive on their exact EDD.
What if my cycle is longer or shorter than 28 days?
This calculator adjusts the EDD for your cycle length. A 35-day cycle shifts the EDD 7 days later, since ovulation occurs around day 21 rather than day 14.
Can I calculate my due date from the date of sex?
Yes — use the Conception Date method and enter the date you believe conception occurred.
How does an IVF due date differ from a natural pregnancy?
In IVF, the date of fertilisation is known precisely. A 5-day blastocyst transfer adds 261 days; a 3-day embryo adds 263 days.
What does "40 weeks pregnant" actually mean?
Pregnancy is counted from the LMP, not conception. "40 weeks" from the LMP is approximately 38 weeks from fertilisation. ACOG defines full term as 39 weeks 0 days through 40 weeks 6 days.
Can my due date change after an ultrasound?
Yes. If an early ultrasound differs from your LMP dates by more than 7 days, your provider may revise the EDD to match the ultrasound measurement.
Is a due date the same as an expected delivery date (EDD)?
Yes — "due date," "EDD," and "expected delivery date" are all used interchangeably for the same estimated birth date.
What is the difference between gestational age and fetal age?
Gestational age counts from the LMP; fetal age counts from conception. They differ by approximately 2 weeks in most pregnancies.
Limitations and When to Call Your Midwife or OB-GYN
This is a general estimation tool, not a medical assessment. It cannot account for irregular cycles, uncertain dates, multiple pregnancies, previous pregnancy complications, or individual health conditions. Always confirm your due date with a healthcare provider and attend all recommended prenatal appointments. Contact your midwife or obstetrician promptly if you experience bleeding, severe abdominal pain, reduced fetal movement, or any other concerning symptoms during pregnancy.
Sources & References (ACOG, NHS, CDC)
- American College of Obstetricians and Gynecologists (ACOG) — Methods for Estimating the Due Date. Committee Opinion No. 700 (acog.org).
- ACOG — Definition of Term Pregnancy. Committee Opinion No. 579 (acog.org).
- NHS — Your antenatal appointments (nhs.uk/pregnancy).
- Centers for Disease Control and Prevention — Pregnancy and Prenatal Care (cdc.gov).