Pregnancy Due Date Calculator
Estimate your due date and track your pregnancy timeline.
How Is the Due Date Calculated?
Pregnancy typically lasts about 40 weeks from the first day of your last menstrual period (LMP). The most common method — Naegele's Rule — adds 280 days (40 weeks) to your LMP to estimate your due date. This calculator also supports calculating from the conception date, in which case 266 days (38 weeks) are added since ovulation/fertilization occurs roughly 14 days into a 28-day cycle.
Naegele's Rule Formula
Due Date = LMP + 280 days (adjusted for cycle length)
If your cycle is longer or shorter than 28 days, the calculator adjusts accordingly: it adds or subtracts the difference from the standard 14-day ovulation assumption.
The Three Trimesters
- First Trimester: Weeks 1–13. Major organs begin to form; the heartbeat starts around week 6.
- Second Trimester: Weeks 14–27. Often the most comfortable period; the anatomy scan happens around week 20.
- Third Trimester: Weeks 28–40. Rapid growth; the baby reaches viability (ability to survive outside the womb) around week 24 and full term at week 37.
Key Milestones Explained
- First Heartbeat (Week 6): The fetal heartbeat becomes detectable on ultrasound.
- Anatomy Scan (Week 20): A detailed ultrasound checks the baby's organs and growth.
- Viability (Week 24): With medical care, a baby born at this stage has a chance of survival.
- Full Term (Week 37): Babies born from week 37 are considered full term and generally healthy.
Example: Calculating a Due Date from LMP
Suppose the first day of your last menstrual period was January 1, and your cycle is 28 days. Using Naegele's Rule, the calculator adds 280 days to January 1, producing an estimated due date of October 8. If your cycle is 35 days instead of 28, the calculator adds 7 extra days (35 − 28 = 7), shifting the due date to October 15 — because ovulation occurs later in a longer cycle. If you are calculating from the conception date instead, the calculator adds 266 days. For a conception date of January 15, the estimated due date would be October 8, roughly two weeks later than the LMP method because the LMP counts from the start of the cycle while conception counts from fertilization. Use this calculator to enter your own dates and see your personalized timeline.
How Cycle Length Affects Your Due Date
Naegele's Rule assumes a 28-day cycle with ovulation on day 14. In reality, cycle lengths vary widely — anywhere from 21 to 35 days is considered normal. Because ovulation typically happens about 14 days before your next period, a longer cycle means ovulation occurs later, which pushes the due date later. A shorter cycle means ovulation happens earlier, moving the due date earlier. This calculator adjusts automatically: for each day your cycle exceeds 28, your due date shifts forward by one day, and vice versa. If your cycles are irregular, the LMP method may be less reliable, and an early ultrasound is the best way to confirm your dates.
LMP vs. Conception Date: Which Should You Use?
The LMP method is the most common because many people remember their last period but not the exact date of conception. However, it assumes a regular 28-day cycle, which not everyone has. The conception date method is more precise if you know when fertilization occurred — for example, if you tracked ovulation or used fertility treatments. The two methods differ by about 14 days because the LMP counts from the start of the cycle, while conception counts from fertilization (roughly day 14). If you are unsure of your conception date but your cycles are irregular, ask your healthcare provider about an early ultrasound, which can date the pregnancy within a few days.
Why Only 5% of Babies Arrive on Their Due Date
A due date is an estimate, not a deadline. Only about 5% of babies are born on their exact due date, while roughly 80% arrive within two weeks before or after. This is because the 280-day average is just a statistical midpoint — individual pregnancies vary based on genetics, first-time versus subsequent pregnancies, maternal health, and other factors. First babies often arrive a few days late, while subsequent pregnancies may come slightly early. Your healthcare provider will monitor your progress and may recommend induction if you go past 41 weeks, as the risk of complications increases with post-term pregnancies. Use this calculator as a planning tool, but stay flexible about your actual delivery date.
Tracking Your Pregnancy Week by Week
This calculator shows your current week, days pregnant, trimester, and a timeline of key milestones. Knowing your week-by-week progress helps you prepare for appointments, screenings, and lifestyle adjustments. For example, the first trimester brings fatigue and morning sickness, the second trimester is often more comfortable and includes the anatomy scan, and the third trimester involves final growth and birth preparation. Revisit the calculator throughout your pregnancy to see how your timeline advances. If your healthcare provider adjusts your due date based on ultrasound measurements, simply update the input date to refresh your milestones.
Frequently Asked Questions
Only about 5% of babies are born on their exact due date. Most arrive within two weeks before or after. An early ultrasound (before 12 weeks) can provide a more accurate estimate than the LMP method alone. Always consult your healthcare provider for the most reliable dating.
Enter your actual average cycle length and the calculator adjusts the due date. Ovulation typically occurs 14 days before your next period, so a longer cycle pushes ovulation (and the due date) later, while a shorter cycle moves it earlier.
The LMP method counts from the first day of your last period (adding 280 days), while the conception method counts from the date of fertilization (adding 266 days). The two differ by about two weeks because ovulation happens roughly 14 days into a typical cycle.
Full term begins at 37 weeks. Babies born between 37 and 42 weeks are considered full term. Born before 37 weeks is preterm, and after 42 weeks is post-term. Most healthcare providers will discuss induction if you go past 41 weeks.