Pregnancy Due Date Calculator: When Is Your Baby Arriving?
!Pregnancy and expecting parents
One of the first questions expecting parents ask is: "When is the baby due?" While due dates are estimates—only about 5% of babies arrive exactly on their due date—knowing your expected delivery date helps you plan and track your pregnancy.
Use our Date Calculator for due date calculations.
How Due Dates Are Calculated
Naegele's Rule
The standard method, used since the 19th century:
``
Due Date = LMP + 280 days (40 weeks)
`
Or equivalently:
- Add 7 days to your LMP date
- Subtract 3 months
- Add 1 year
- Add 7 days: March 8
- Subtract 3 months: December 8
- Due date: December 8, 2025
From Conception Date
If you know your conception date:
`
Due Date = Conception Date + 266 days (38 weeks)
`
Conception typically occurs about 2 weeks after LMP.
The 40-Week Myth
Pregnancy is counted from the first day of your last menstrual period (LMP), not from conception. This means:
- Weeks 1-2: Not actually pregnant yet (pre-ovulation)
- Week 2-3: Conception occurs
- Week 40: Due date
Actual pregnancy duration from conception: ~38 weeks
Pregnancy Timeline by Trimester
First Trimester (Weeks 1-13)
Week Development Week 4 Implantation complete, positive test possible Week 6 Heart begins beating Week 8 All major organs forming Week 10 Embryo officially becomes fetus Week 12 First ultrasound typically scheduled
Second Trimester (Weeks 14-27)
Week Development Week 16 Baby can hear sounds Week 18-20 May feel first movements ("quickening") Week 20 Anatomy scan ultrasound Week 24 Viability milestone Week 27 Eyes can open
Third Trimester (Weeks 28-40)
Week Development Week 28 Brain developing rapidly Week 32 Baby gaining weight quickly Week 36 Considered "late preterm" if born now Week 37 Full term begins Week 39-40 Optimal delivery window Week 41+ Considered post-term
Due Date Accuracy
By Calculation Method
Method Accuracy LMP calculation ±2 weeks Early ultrasound (6-9 weeks) ±3-5 days First trimester ultrasound ±5-7 days Second trimester ultrasound ±1-2 weeks
First trimester ultrasound is most accurate for dating.
Reality of Due Dates
- Only ~5% of babies arrive on the exact due date
- Most babies arrive within 2 weeks before or after
- Full term is now defined as 39-40 weeks
- 41+ weeks may require induction discussion
Important Dates to Calculate
Key Milestones
Event Calculation End of 1st trimester LMP + 13 weeks Gender possibly visible LMP + 16-20 weeks Anatomy scan LMP + 18-22 weeks Viability LMP + 24 weeks Full term begins LMP + 37 weeks Due date LMP + 40 weeks
Week-by-Week Example
LMP: January 1, 2025
Milestone Date 12 weeks March 26, 2025 20 weeks May 21, 2025 28 weeks July 16, 2025 37 weeks (full term) September 24, 2025 40 weeks (due date) October 15, 2025
Factors Affecting Due Date
Can Change Your Date
- First trimester ultrasound measurements
- Irregular menstrual cycles
- Conception date (if known precisely, e.g., IVF)
- Baby's growth patterns
Don't Change Your Date
- Baby measuring large or small later in pregnancy
- Symptoms or feelings
- Family history of early/late delivery
Full Term vs. Preterm
Classification Weeks Extremely preterm <28 weeks Very preterm 28-32 weeks Moderate preterm 32-34 weeks Late preterm 34-36 weeks Early term 37-38 weeks Full term 39-40 weeks Late term 41 weeks Post-term 42+ weeks
Optimal delivery: 39-40 weeks when possible.
IVF Due Date Calculation
For IVF pregnancies:
From Egg Retrieval
`
Due Date = Egg Retrieval Date + 266 days (38 weeks)
`
From Embryo Transfer
Transfer Type Add to Transfer Date Day 3 embryo 263 days Day 5 embryo (blastocyst) 261 days
From Assigned LMP
Your clinic may assign an LMP for consistency:
`
Assigned LMP = Egg Retrieval - 14 days
``
When to Worry
Contact Your Doctor If
- Bleeding at any stage
- Severe cramping
- Decreased fetal movement (after 28 weeks)
- Signs of preterm labor before 37 weeks
- Water breaking before 37 weeks
Normal Variations
- Due date range: 37-42 weeks is normal
- Size variations: babies grow at different rates
- Symptom timing varies between individuals
Preparing for Your Due Date
6-8 Weeks Before
- Hospital bag prepared
- Pediatrician selected
- Birth plan discussed
4 Weeks Before
- Car seat installed
- Maternity leave arranged
- Nursery ready
At Due Date
- Be patient—baby comes when ready
- Know signs of labor
- Stay in contact with provider
Frequently Asked Questions
Can my due date change?
Yes, typically only in the first trimester based on ultrasound measurements. Later changes are rare.
What if my periods are irregular?
Early ultrasound dating is more accurate than LMP for irregular cycles.
How is the due date different from the conception date?
Due date is 40 weeks from LMP; conception is about 38 weeks before delivery. The 2-week difference accounts for pre-ovulation time.
Should I schedule a C-section for my due date?
If planned cesarean is needed, it's typically scheduled at 39 weeks unless medical reasons require earlier delivery.
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