Congratulations! Whether you just found out you’re pregnant or you’re a few weeks in, one of the very first questions on every expecting parent’s mind is: “When is my baby due?”
Our free Pregnancy Due Date Calculator gives you an estimated delivery date in seconds — based on your last menstrual period, conception date, ultrasound, or IVF transfer date. No OB appointment required for a first estimate.
How to Use the Due Date Calculator
This tool supports four different calculation methods. Use whichever one applies to your situation:
Method 1 — Last Menstrual Period (LMP) Enter the first day of your last period and your average cycle length. This is the most common method used by OB-GYNs across the US.
Method 2 — Conception Date If you know approximately when conception occurred, enter that date directly. The calculator will adjust accordingly.
Method 3 — Ultrasound Date Enter your ultrasound date and how far along you were at that point. This is often the most accurate method, especially in early pregnancy.
Method 4 — IVF Transfer Date If you conceived through in vitro fertilization, enter your embryo transfer date and whether it was a Day 3 or Day 5 transfer.
Hit Calculate — your estimated due date, current gestational age, and trimester information will appear instantly.
How Is a Due Date Calculated? (The Science Behind It)
The Standard Method: Naegele’s Rule
The most widely used method for estimating a due date is called Naegele’s Rule, developed by German obstetrician Franz Karl Naegele in the early 1800s. Despite its age, it remains the standard method used by healthcare providers throughout the United States today.
The formula:
Due Date = First Day of Last Menstrual Period + 280 Days (40 weeks)
Or using the shortcut version:
Add 1 year, subtract 3 months, add 7 days to the first day of your LMP.
Example: Last menstrual period: August 1, 2024 Add 1 year → August 1, 2025 Subtract 3 months → May 1, 2025 Add 7 days → Due Date: May 8, 2025
This formula assumes a 28-day menstrual cycle with ovulation on Day 14. If your cycle is longer or shorter, the calculator automatically adjusts.
The Four Methods Explained in Detail
1. Last Menstrual Period (LMP)
This is the default method used by virtually all OB-GYNs and midwives in the United States. Even though conception hasn’t occurred yet at the start of your last period, the LMP is used as the universal starting point because it’s a known, trackable date — unlike the exact moment of conception.
Why 40 weeks? A typical pregnancy lasts 280 days from the LMP — or about 40 weeks, divided into three trimesters. Conception actually happens around Week 2, meaning the baby is biologically about 38 weeks old at birth — but the pregnancy is counted as 40 weeks from LMP.
What if my cycle isn’t 28 days? No problem. If your cycle is, say, 32 days instead of 28, ovulation likely occurs around Day 18 instead of Day 14 — so the calculator shifts the due date by 4 days accordingly.
2. Conception Date
If you were tracking ovulation through basal body temperature, ovulation predictor kits (OPKs), or fertility monitoring, you may know your approximate conception date.
The due date based on conception is calculated as:
Due Date = Conception Date + 266 Days (38 weeks)
Note that this should produce nearly the same result as the LMP method — the difference is approximately 2 weeks (14 days), which is the average gap between the start of the last period and ovulation.
3. First Trimester Ultrasound
An ultrasound, particularly one performed between 8 and 14 weeks, is considered the most accurate method for estimating gestational age and due date. The reason? Early on, all fetuses grow at nearly the same rate, so measuring the fetus (called a crown-rump length, or CRL, measurement) gives a very precise age estimate.
If your ultrasound-based due date differs from your LMP-based due date by more than 5–7 days, your OB may revise your official due date to match the ultrasound.
After 14 weeks, ultrasound accuracy decreases slightly because babies begin to grow at more individual rates.
4. IVF Transfer Date
When pregnancy occurs through in vitro fertilization (IVF), the due date calculation is actually more precise than natural conception — because the exact transfer date is known.
The calculation depends on the embryo’s age at transfer:
| Transfer Type | Days Added to Transfer Date | Equivalent LMP Date |
|---|---|---|
| Day 3 embryo transfer | +263 days | Transfer date − 17 days |
| Day 5 embryo transfer | +261 days | Transfer date − 19 days |
Your fertility clinic will typically calculate your due date for you, but our calculator handles this automatically.
Your Pregnancy Week by Week: What to Expect
First Trimester (Weeks 1–12)
This is the most critical period for fetal development. Major organs and body systems begin forming. Most women experience morning sickness, fatigue, and breast tenderness during this stage. The risk of miscarriage is highest in the first trimester.
Key milestones:
- Week 6: Heartbeat detectable on ultrasound
- Week 8: Embryo becomes a fetus; limb buds present
- Week 10: Fingernails begin to form
- Week 12: Risk of miscarriage drops significantly; first trimester screening performed
Second Trimester (Weeks 13–26)
Often called the “honeymoon trimester” — many women feel much better during this phase. The baby grows rapidly, and movement becomes noticeable around Weeks 18–22.
Key milestones:
- Week 16: Baby can make facial expressions
- Week 18–20: Anatomy ultrasound performed; sex of baby can usually be determined
- Week 24: Considered the threshold of viability outside the womb
- Week 26: Baby’s lungs begin producing surfactant
Third Trimester (Weeks 27–40)
The baby gains significant weight and prepares for birth. The mother’s body changes substantially to accommodate the growing baby.
Key milestones:
- Week 28: Kick counts begin; Group B Strep screening scheduled
- Week 32: Baby typically settles into head-down position
- Week 36: Weekly OB appointments begin
- Week 37: Baby is considered early term
- Week 39–40: Full term; optimal delivery window
- Week 41–42: Late term; induction may be discussed
Understanding Pregnancy Terms: Early, Full, Late, and Post-Term
Many people assume any baby born at 37+ weeks is “full term.” That’s not quite accurate — here’s how the American College of Obstetricians and Gynecologists (ACOG) officially defines these terms:
| Term | Gestational Age | Notes |
|---|---|---|
| Preterm | Before 37 weeks | May require NICU support |
| Early Term | 37–38 weeks, 6 days | Generally healthy; some risks remain |
| Full Term | 39–40 weeks, 6 days | Optimal delivery window |
| Late Term | 41–41 weeks, 6 days | Monitoring increases |
| Post-Term | 42 weeks or beyond | Induction typically recommended |
Only 4% of babies are actually born on their exact estimated due date. About 80% of births happen within 2 weeks before or after the due date. Think of the due date as the center of a window, not a specific appointment.
How Accurate Is a Due Date Calculator?
This is one of the most common questions expectant parents ask. The honest answer: pretty accurate, but not perfect.
Here’s what affects due date accuracy:
Cycle length variability — Naegele’s Rule assumes a 28-day cycle. If your cycles are irregular or longer/shorter, your LMP-based due date may be off by several days.
Uncertain LMP — If you can’t recall the exact first day of your last period, the LMP method loses precision.
First trimester ultrasound — This is the gold standard for accuracy. Studies show first-trimester ultrasounds are accurate to within ±5 days in most cases.
Later ultrasounds — Less accurate for gestational dating because babies grow at increasingly individual rates after the first trimester.
IVF pregnancies — Most accurate of all since the exact transfer date is documented.
Bottom line: Your due date is an estimate. Only about 1 in 25 babies arrives on the exact date predicted. Use it as a planning guide, not a guarantee.
What If You Go Past Your Due Date?
Going beyond 40 weeks is more common than most people realize — especially for first-time mothers, who on average deliver at 40 weeks and 5 days.
Here’s what typically happens:
- 40–41 weeks: Your OB will monitor you with non-stress tests (NSTs) and biophysical profiles to ensure baby is doing well.
- 41–42 weeks: Many OBs recommend discussing induction at this point.
- 42+ weeks: Most providers will recommend induction or cesarean delivery to avoid placental insufficiency, meconium complications, or other risks associated with post-term pregnancies.
If you have a high-risk pregnancy, your provider may recommend induction before 40 weeks. Always follow the guidance of your healthcare provider.
Pregnancy Due Date by Conception Month (Quick Reference)
| Last Period Start Month | Estimated Due Date Month |
|---|---|
| January | October |
| February | November |
| March | December |
| April | January |
| May | February |
| June | March |
| July | April |
| August | May |
| September | June |
| October | July |
| November | August |
| December | September |
This is a rough guide based on a 40-week pregnancy. Use the calculator above for your specific due date.
Twins and Multiple Pregnancies
If you’re expecting twins or multiples, the due date calculation works the same way — but the expected delivery date is often earlier:
| Pregnancy Type | Average Delivery Gestational Age |
|---|---|
| Singleton (one baby) | 40 weeks |
| Fraternal (dizygotic) twins | ~37–38 weeks |
| Identical (monochorionic) twins | ~36–37 weeks |
| Triplets | ~32–35 weeks |
Multiple pregnancies carry a higher risk of preterm birth, which is why twin and triplet pregnancies are monitored more closely throughout.
Frequently Asked Questions (FAQ)
How do I calculate my due date from my last period?
Add 280 days (40 weeks) to the first day of your last menstrual period. Our calculator does this automatically — just enter your LMP date and cycle length.
How accurate are due date calculators?
LMP-based calculators are accurate to within about 1–2 weeks. First-trimester ultrasound is the most accurate method, typically accurate within ±5 days. Only about 4% of babies are born on their exact due date.
What does LMP stand for in pregnancy?
LMP stands for Last Menstrual Period. It refers to the first day of your most recent period before becoming pregnant. It’s the universal starting point for pregnancy dating used by healthcare providers in the US.
Can my due date change after an ultrasound?
Yes. If your ultrasound shows a gestational age that differs from your LMP-based estimate by more than 5–7 days, your OB may officially revise your due date. This is most common when cycles are irregular or LMP date is uncertain.
What is the difference between gestational age and fetal age?
Gestational age is calculated from the first day of your last menstrual period and is the standard used in clinical settings. Fetal age (also called embryonic age) is counted from the actual day of conception, which is typically about 2 weeks after LMP. So a 10-week-gestational-age fetus is about 8 weeks old in fetal age.
How is an IVF due date calculated?
For a Day 5 embryo transfer, add 261 days. For a Day 3 transfer, add 263 days. Because the exact transfer date is known, IVF due dates tend to be more precise than LMP-based estimates.
What happens if I’m overdue?
Most OBs begin additional monitoring (non-stress tests, fluid checks) at 41 weeks. Many recommend induction between 41 and 42 weeks to reduce the small but real risk of placental insufficiency and other complications in post-term pregnancies.
How many weeks pregnant am I?
Subtract the first day of your last menstrual period from today’s date, then divide by 7. Our calculator shows your current gestational age automatically along with your due date.
Is a 38-week baby considered full term?
No. According to ACOG, full term is defined as 39–40 weeks and 6 days. A baby born at 38 weeks is considered “early term.” While most early term babies are healthy, they can face slightly higher risks than full-term babies, particularly with lung development and feeding.
What is a normal range for a full-term birth?
Births occurring between 37 and 42 weeks are considered within the normal range. The optimal window — associated with the best outcomes — is 39 to 41 weeks.
Related Calculators You’ll Find Helpful
- Age Calculator — Calculate your baby’s age from their birth date as they grow
- Date Calculator — Count down the days until your due date
- Time Calculator — Track hours and minutes for labor timing
- BMI Calculator — Monitor healthy weight changes during pregnancy
- Ovulation Calculator — Identify your most fertile days for conception planning
This Pregnancy Due Date Calculator is intended for informational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always consult your OB-GYN, midwife, or healthcare provider for personalized pregnancy care and an official due date.