Due Date Calculator – When Will Baby Arrive?
Pregnancy is a journey of 40 weeks, but the math isn't always simple. Calculate your official Estimated Due Date (EDD) using the same logic as your doctor.
Calculation Method
Your Due Date
"When are you due?" is the first question everyone asks. But a "Due Date" is actually a misnomer—it is a Due Window.
Only about **4%** of babies are born on their exact 40-week date. This tool helps you plan for the arrival window, schedule prenatal screening tests, and track your trimester milestones.
LMP vs. Ultrasound
Doctors initially date pregnancy by your Last Menstrual Period (LMP). However, a "Dating Scan" (First Trimester Ultrasound) typically trumps this date if the baby measures more than 5-7 days different from the LMP math.
Gestational Age
Note that "Gestational Age" (how far along you are) includes the 2 weeks before you conceived. So, the moment you get a positive test, you are technically already "4 weeks pregnant".
Defining "Full Term"
Historically, any baby born between 37 and 42 weeks was considered "term". In 2013, ACOG (American College of Obstetricians and Gynecologists) refined these definitions because research showed babies born at 39 weeks do better than those at 37 weeks.
The "2-Week Rule" of Ovulation
Standard calculators assume you ovulate on Day 14 of a 28-day cycle. If your cycle is 35 days, you likely ovulate on Day 21.
Impact: If you have long cycles, your LMP-based due date might be too early. If you ovulated late, your baby is actually younger than the math suggests. This is why early ultrasounds are crucial—they measure the baby's crown-rump length (CRL) to determine the true biological age regardless of your period math.
Planning by Trimester
Critical Milestones Timeline
Focus on prenatal vitamins (Folic Acid). The most critical development happens here. Schedule your "Dating Scan" around Week 8.
The "Honey-moon Phase". Energy returns. Anatomy scan (gender reveal) happens around Week 20. Feel baby move!
Growth mode. Baby gains fat. Finalize birth plan, pack hospital bag, and install car seat by Week 36.
How Doctors Calculate EDD
Our calculator uses Naegele's Rule, the standard formula used by obstetricians worldwide to determine the Estimated Date of Delivery (EDD).
Technically, this adds 280 days (40 weeks) to the first day of your Last Menstrual Period (LMP). If you choose the "Conception Date" method, we add 266 days (38 weeks), acknowledging that conception typically occurs during ovulation, roughly 2 weeks after your period starts.
The "Missing Week" Mystery
Standard 28-Day Cycle
LMP: January 1st
Ovulation: January 15th (Day 14)
Actual Pregnancy Age: Matches Calculator exactly.
Accuracy: High (95%)
Long 35-Day Cycle
LMP: January 1st
Ovulation: January 22nd (Day 21) - "Late"
Result: Calculator says Due Oct 8th, but baby is actually due Oct 15th.
Accuracy: Low ( Ultrasound needed to correct date )
Related Tools
Why did my doctor change my due date?
This is common. If your first-trimester ultrasound measurements differ from your LMP math by more than 5-7 days, the ultrasound date is generally adopted because fetal growth is extremely uniform in early pregnancy, making it a better ruler than menstrual history.
Does IVF change the due date?
Yes, IVF dates are precise because the exact moment of conception (fertilization) is known. We essentially take the transfer date and subtract 2 weeks and 3 days (for Day 3 transfer) or 2 weeks and 5 days (for Day 5 blastocyst) to get a theoretical LMP.
What are the chances I give birth on my due date?
Roughly 4-5%. About 80% of women deliver between 37 and 42 weeks. First-time moms are statistically more likely to go past 40 weeks than moms who have given birth before.
How do I calculate conception date from due date?
It's simple subtraction. Take your Due Date and count BACKWARDS 266 days (38 weeks). This gives you the approximate date ovulation/fertilization occurred.
Is calculating by conception more accurate?
If you were tracking ovulation with strips or temperature (BBT) and know the exact date, YES. Conception-based calculation removes the error variable of 'when did I actually ovulate?'.
Pregnancy Terminology
LMP
Last Menstrual Period. Specifically, the FIRST day of bleeding of your last cycle. This is Day 0 of the pregnancy clock.
EDD
Estimated Date of Delivery. The 40-week marker. Also referred to as EDB (Estimated Date of Birth).
Gestation
The period of carrying the baby. Interestly, elephants have a gestation of 22 months, making 9 months seem quick!
Viability
The point at which a baby could survive outside the womb (with NICU help). Currently considered around 24 weeks.
About This Calculator
We utilize the American College of Obstetricians and Gynecologists (ACOG) standard formula for dating pregnancy. This assumes a 280-day gestation from LMP. While highly accurate for standard cycles, individual variations occur. Our content is reviewed against the latest 2025/2026 obstetrics guidelines.