PregnancyDue Date Calculator — When Is Your Baby Due?
Enter the first day of your last menstrual period and get your estimated due date, current pregnancy week, trimester, and countdown to delivery — instantly and free.
Pregnancy Due Date Calculator
TL;DR
- A pregnancy due date is an estimated delivery date, usually calculated from the first day of your last menstrual period.
- The standard calculation adds 280 days, or 40 weeks, to your LMP and adjusts for cycle length.
- Only about 4% of babies arrive on their exact due date; most arrive within a wider birth window.
- A first-trimester ultrasound is usually the most accurate way to confirm or adjust dating, especially if your cycles are irregular.
- Use this calculator as a planning tool, then confirm your date with your doctor, midwife, or fertility clinic.
How to Use the Baby Due Date Calculator
- Enter your LMP date: Use the first day bleeding began during your last menstrual period, not the last day of bleeding.
- Select your average cycle length: If you are not sure, 28 days is a reasonable default.
- Calculate your estimate: The tool estimates your due date, current pregnancy week, trimester, days remaining, and pregnancy progress.
- Compare with medical dating: Bring the result to your first prenatal visit so your provider can compare it with ultrasound measurements and your health history.
- Track your pregnancy: Once your date is estimated or confirmed, follow symptoms, appointments, and fetal development with a pregnancy tracker or week-by-week guide.
How Is a Due Date Calculated?
The most common method is Naegele’s rule: add 280 days, or 40 weeks, to the first day of your last menstrual period (LMP). This gives an estimated due date, often shortened to EDD.
Naegele’s rule assumes a 28-day menstrual cycle with ovulation around day 14. If your cycle is longer, ovulation may have happened later, so the estimated due date may move forward. If your cycle is shorter, ovulation may have happened earlier, so the estimate may move back. The calculator above adjusts for cycle length within the available range.
Pregnancy is dated from the first day of your last period, not from conception. That means gestational age includes roughly two weeks before conception. This is why someone may be called “4 weeks pregnant” only about two weeks after ovulation.
How Accurate Is an Estimated Due Date?
An estimated delivery date is not a scheduled arrival day. It is best understood as the center of a normal birth window.
Only about 4% of babies are born on their exact due date. Many babies arrive between 37 and 42 weeks, with clinical decisions depending on your health, your baby’s wellbeing, and local guidelines. The NHS notes that pregnancy usually lasts about 40 weeks from the first day of the last period, but birth may happen before or after that date; see its overview of working out your due date.
Several factors can affect due date accuracy:
- Irregular cycles: LMP-based dating is less precise if your periods are not predictable.
- Ovulation timing: Ovulation does not always happen on day 14, even with a regular cycle.
- Uncertain period dates: If you are unsure when your last period started, the estimate may shift.
- Pregnancy history: First babies may arrive a few days later than the estimated due date on average.
If your due date changes after a scan, it does not mean you did anything wrong. It usually means your care team has more accurate information.
Last Menstrual Period vs. Conception Date
Due date calculators usually ask for the first day of your last menstrual period because most people remember when bleeding started more reliably than the exact day of fertilization.
Conception usually happens around ovulation. In a 28-day cycle, ovulation is often around day 14; in a 32-day cycle, it may be closer to day 18. Sperm can survive in the reproductive tract for up to five days, so the exact moment of fertilization is often hard to pinpoint.
If you know your conception date — for example, from ovulation tracking or fertility treatment — a due date can be estimated by adding 266 days, or 38 weeks. Fertility clinics may date pregnancy using embryo age and transfer date, then convert to gestational age.
If you are tracking cycle awareness before pregnancy, compare options in our guide to the best ovulation app.
What If You Do Not Know Your LMP?
If you do not know the date of your last period, your due date can still be estimated. The most helpful next step is usually an early prenatal appointment and, when appropriate, a first-trimester ultrasound.
This is common. You may have irregular periods, recent birth control changes, postpartum cycles, breastfeeding, spotting that looked like a period, or simply no reason to track dates before a positive test.
Your provider may ask about the date of your first positive pregnancy test, cycle pattern, symptoms, ovulation tracking, or fertility treatment details. Later clues, such as fundal height or when you first feel movement, can support the picture but are less precise than early ultrasound.
If you are early in pregnancy and feeling overwhelmed, our first trimester must-haves guide covers practical next steps.
Due Date by Ultrasound
Early ultrasound is usually the most accurate way to confirm or adjust pregnancy dating when LMP-based dating is uncertain. In the first trimester, embryo growth is predictable enough that crown-rump length can estimate gestational age within about 5–7 days.
Ultrasound dating accuracy changes as pregnancy progresses:
- Before 9 weeks: about ±5 days
- 9 to 13 weeks: about ±7 days
- 14 to 20 weeks: about ±10 days
- After 20 weeks: about ±14–21 days because babies grow at different rates
The American College of Obstetricians and Gynecologists explains that first-trimester ultrasound is especially useful for establishing or confirming gestational age in its guidance on methods for estimating the due date.
Once your due date is confirmed, it generally should not be changed repeatedly unless your provider has a clinical reason.
What Is a Full-Term Pregnancy?
A full-term pregnancy is not just one day at 40 weeks. Clinically, birth timing is described in ranges because babies develop and arrive on individual timelines.
- Early term: 37 weeks 0 days through 38 weeks 6 days
- Full term: 39 weeks 0 days through 40 weeks 6 days
- Late term: 41 weeks 0 days through 41 weeks 6 days
- Post-term: 42 weeks 0 days and beyond
These categories help providers discuss monitoring, induction, and risks, but they do not predict your personal birth timing. As your estimate gets closer, learning the stages of labor can make the waiting feel less mysterious.
Week-by-Week Pregnancy Tracking After Your Due Date Estimate
Once your date is estimated or confirmed, week-by-week tracking helps connect the calendar to what is happening in your body. It can show fetal development, common symptoms, appointment timing, and trimester milestones.
First Trimester: Weeks 1–12
Implantation usually happens around week 4. By week 6, cardiac activity may be visible on ultrasound. By week 8, major organs are forming. Common symptoms include nausea, fatigue, breast tenderness, and food aversions.
Second Trimester: Weeks 13–26
Nausea often improves, energy may return, and the baby bump may become more noticeable. The anatomy scan commonly happens around 18–22 weeks. Many people feel first movements, also called quickening, between weeks 16 and 25.
Third Trimester: Weeks 27–40
The baby gains weight, the lungs continue maturing, and the baby often settles head-down before birth. Common symptoms include back pain, frequent urination, Braxton Hicks contractions, and sleep difficulty.
Use tracking as a gentle map, then bring medical questions to your doctor or midwife. Pregnancy App also offers a pregnancy week-by-week overview for plain-language guidance from early pregnancy through the final weeks.
Tracking Baby Movement and Preparing for Labor
Your estimated due date becomes more practical as movement, appointments, and third-trimester planning begin. Later in pregnancy, tracking patterns can help you notice what is normal for your baby.
Many providers recommend paying attention to fetal movement in the third trimester, especially if you sense a change from your baby’s usual rhythm. Contact your maternity unit or healthcare provider if movement is reduced, unusual, or worrying. For a simple way to record movement patterns, see the baby kick counter.
As you approach the final weeks, prepare gradually: know how to reach your provider, pack essentials, discuss preferences with your birth partner, and learn the difference between practice contractions and labor contractions. A contraction timer can help you record contraction length and spacing before calling your care team. If you are feeling nervous, our guide on how to prepare for labor offers practical steps for hospital, birth center, and home-birth plans.
Due Date Apps and Pregnancy Tools Compared
Pregnancy dating tools are most useful when they explain the “why” behind the estimate, not just the final date. The best option depends on whether you want a simple calculator, a full tracker, community features, or birth-preparation tools.
| Tool | Best for | Notable limitation |
|---|---|---|
| Pregnancy App | Comparing pregnancy calculators, trackers, timers, and birth tools in one place | It is a guide site, not a substitute for clinical dating |
| Flo | Cycle history and fertility-to-pregnancy continuity | Predictions depend heavily on accurate cycle data |
| Ovia Pregnancy | Pregnancy tracking, symptom logs, and baby-size updates | Can feel busy if you only want a quick estimate |
| What to Expect | Week-by-week articles and community discussion | Community content may not match your medical situation |
If you are choosing a broader tool, our best pregnancy tracker app comparison may help.
Frequently Asked Questions
How is a pregnancy due date calculated?
The most common method is Naegele’s rule. You add 280 days, or 40 weeks, to the first day of your last menstrual period. If your cycle is longer or shorter than 28 days, the estimate is adjusted by adding or subtracting the difference.
How accurate is a due date calculator?
A due date calculator gives an estimate, not a guarantee. Only about 4% of babies are born on their exact due date. A first-trimester ultrasound is usually more accurate than an LMP-based calculator when period dates or ovulation timing are uncertain.
Can my due date change?
Yes. Your provider may adjust your due date after an early ultrasound if the baby’s measurements differ from the LMP-based estimate by enough to meet clinical criteria. Once a due date is confirmed, it usually is not changed again.
What is the difference between gestational age and fetal age?
Gestational age is counted from the first day of your last menstrual period. Fetal age, sometimes called embryonic age, is counted from the estimated date of conception. Medical providers usually use gestational age.
Is a due date based on conception more accurate?
It can be more accurate if the conception date is truly known, such as with fertility treatment or careful ovulation tracking. For many pregnancies, the exact conception date is uncertain because ovulation can vary and sperm can survive for several days.
Does cycle length affect my due date?
Yes. A longer cycle may mean ovulation happened later, which can shift the due date forward. A shorter cycle may mean ovulation happened earlier, which can shift the due date earlier.
Key Takeaway for Your Pregnancy Timeline
A due date calculator gives you a useful starting point, but your pregnancy timeline should be confirmed and interpreted with clinical care. Treat the date as a planning anchor, not a deadline.
If you know your LMP, cycle length, conception date, or IVF details, you can estimate your timing quickly. If you do not know those details, an early ultrasound can often clarify gestational age. From there, tracking week by week, preparing for appointments, and learning late-pregnancy signs can make the months feel more manageable.
Limitations & Safety
- This calculator is educational: It estimates timing but cannot diagnose pregnancy health, predict your actual birth day, or replace prenatal care.
- Irregular cycles, uncertain bleeding, IVF, and fertility treatment need clinical interpretation: Use the date your provider or fertility clinic gives you.
- Ultrasound timing matters: Early scans are generally more accurate for dating than later scans.
- Do not use this tool to schedule medical decisions: Induction, cesarean timing, medication, and testing decisions should be made with your healthcare provider.
- Seek care promptly for concerning symptoms: Heavy bleeding, severe pain, fever, fluid leaking, reduced fetal movement, or severe headache should be assessed right away.
This is not medical advice. Consult your healthcare provider.