Pregnancy Calculator

Pregnancy is typically calculated from the first day of your Last Menstrual Period (LMP), not from conception. This means you're considered "pregnant" for about 2 weeks before you actually conceive.

Key Pregnancy Terminology:

• LMP: Last Menstrual Period - the starting point for pregnancy calculation
• Gestational Age: Time since LMP (pregnancy weeks/months)
• Fetal Age: Actual age of the developing baby (about 2 weeks less than gestational)
• EDD/Due Date: Estimated Date of Delivery (40 weeks from LMP)
• Trimesters: Pregnancy divided into three 13-week periods

Important Statistics:

• Only about 5% of babies arrive on their due date
• Most babies are born within 2 weeks of the due date
• Full term is now defined as 39-40 weeks
• Average first pregnancy goes slightly past due date
• Second+ pregnancies average slightly earlier

The standard method for calculating due date is Naegele's Rule, developed in the early 1800s and still used today:

Pregnancy is divided into three trimesters, each with distinct developmental milestones:

Pregnancy Term Definitions:

• Preterm: Before 37 weeks
• Early term: 37-38 weeks
• Full term: 39-40 weeks
• Late term: 41 weeks
• Post-term: 42+ weeks

Our calculator estimates your due date and tracks pregnancy milestones:

1. Enter Starting Point:
   • First day of your last menstrual period (most common), OR
   • Conception date (if known), OR
   • IVF transfer date (select embryo age)
2. Enter Average Cycle Length (optional): Default is 28 days. If your cycles are longer/shorter, this adjusts the calculation.
3. View Results:
   • Estimated due date
   • Current weeks and days pregnant
   • Current trimester
   • Key milestone dates

Important Notes:

• This is an estimate - ultrasound dating is more accurate
• Due dates may be adjusted based on early ultrasound
• Consult your healthcare provider for official dating

Important dates and milestones throughout pregnancy:

First Trimester:

• Week 4: Implantation, pregnancy test positive
• Week 6: Heartbeat may be detected on ultrasound
• Week 8: All major organs beginning to form
• Week 10-13: First prenatal visit, dating scan, NIPT screening available
• Week 12: Risk of miscarriage drops significantly

Second Trimester:

• Week 14: Energy often returns, morning sickness usually eases
• Week 16-20: First movements felt ("quickening")
• Week 18-22: Anatomy scan (detailed ultrasound)
• Week 20: Halfway point
• Week 24: Viability milestone - survival outside womb possible with intensive care

Third Trimester:

• Week 28: Third trimester begins, more frequent prenatal visits
• Week 32: Baby likely in head-down position
• Week 36: Full term approaching, Group B strep test
• Week 37: Early term begins
• Week 39-40: Full term - optimal time for delivery

Several methods are used to estimate due date, each with different accuracy:

1. Last Menstrual Period (LMP)

• Most common starting point
• Assumes regular 28-day cycles and ovulation on day 14
• May be inaccurate if cycles are irregular or LMP uncertain
• Accuracy: ±2-3 weeks

2. Early Ultrasound (Most Accurate)

• First trimester (6-13 weeks) ultrasound is most accurate
• Measures crown-rump length
• Accuracy: ±3-5 days in first trimester
• Less accurate later in pregnancy (±2-3 weeks by third trimester)

3. Conception Date

• If known precisely (IVF, tracked ovulation)
• Add 266 days (38 weeks) to conception date
• Most accurate if date is certain

4. IVF Transfer Date

• Most precise dating method
• Add 266 days minus embryo age at transfer
• 3-day embryo: Add 263 days
• 5-day embryo: Add 261 days

If ultrasound dating differs from LMP dating by more than a week in the first trimester, the ultrasound date is typically used.

Many factors influence when you'll actually deliver:

Factors That May Lead to Earlier Delivery:

• Multiple pregnancies (twins, triplets)
• History of preterm birth
• Certain medical conditions
• Uterine or cervical abnormalities
• Placental issues

Factors Associated with Longer Pregnancy:

• First pregnancy (average 3-5 days past due)
• Family history of long pregnancies
• Male baby (slightly longer on average)
• Higher BMI before pregnancy

What's Normal:

• 80% of babies are born between 38-42 weeks
• Only 5% arrive on their exact due date
• First-time mothers average 41 weeks + 1 day
• Second+ pregnancies average slightly earlier

Medical Considerations:

Sometimes delivery is recommended before the due date for medical reasons (preeclampsia, gestational diabetes, growth concerns). Your healthcare provider will advise if this applies to you.