How Pregnancy Due Dates Are Calculated
The Standard Method (Naegele's Rule)
Healthcare providers use Naegele's Rule, developed in the 1800s but still accurate today. The calculation is based on the first day of your last menstrual period (LMP), not conception date, because most women remember their last period but not the exact conception day.
Due Date Calculation: 1. Take the first day of your last period (LMP) 2. Add 7 days 3. Subtract 3 months 4. Add 1 year (if necessary) Example: LMP = January 15, 2026 + 7 days = January 22, 2026 - 3 months = October 22, 2025 + 1 year = October 22, 2026 (Due Date) Alternative: LMP + 280 days (40 weeks)
Why 40 Weeks?
Pregnancy is counted as 40 weeks from your last menstrual period, even though conception typically occurs 2 weeks after your period starts (during ovulation). This means you're "pregnant" for 2 weeks before conception actually happens. This is standard medical practice because most women don't know their exact conception date, but they do know their last period.
Full term pregnancy ranges:
- Early term: 37-38 weeks (baby is developed but some benefits from waiting)
- Full term: 39-40 weeks (ideal timing)
- Late term: 41 weeks (still normal, closer monitoring)
- Post-term: 42+ weeks (usually induces labor to prevent complications)
How to Use This Pregnancy Calculator
- Select calculation method: By last period (most common) or by conception date if you know it
- Enter your last menstrual period date: The first day of your last normal period (not spotting)
- Or enter conception date: If you know when you ovulated/conceived (from tracking or IVF)
- Enter average cycle length: Usually 28 days, but can be 21-35 days (affects due date for irregular cycles)
- Click Calculate: See your due date, current week of pregnancy, trimester, and key milestone dates
Note: Your healthcare provider may adjust your due date after an early ultrasound (dating ultrasound at 8-13 weeks is most accurate, within 3-5 days). Ultrasounds measure baby's size to confirm or adjust the due date calculated from your last period.
Understanding Your Pregnancy Timeline
First Trimester (Weeks 1-13)
What's happening: All major organs form during these crucial first 13 weeks. Your baby goes from a cluster of cells to a fully formed fetus with beating heart, brain, limbs, and organs.
Key milestones:
- Week 5-6: Heartbeat detectable on ultrasound
- Week 8: Baby is now called a fetus (no longer an embryo), all organs present
- Week 12: Risk of miscarriage drops significantly (from 15% to 1-5%)
- Week 13: End of first trimester, most morning sickness subsides
Common symptoms: Morning sickness (60-70% of women), extreme fatigue, frequent urination, breast tenderness, food aversions. These usually improve after week 12-14.
Second Trimester (Weeks 14-27)
What's happening: Often called the "honeymoon period" of pregnancy—energy returns, morning sickness fades, you start showing, and you can feel baby move. Baby is growing rapidly and developing fat stores.
Key milestones:
- Week 16-20: Feel baby move for first time (flutters, then kicks)
- Week 18-22: Anatomy scan ultrasound (can find out baby's sex if desired)
- Week 24: Viability milestone (baby has 50%+ survival chance if born, though NICU care would be needed)
- Week 28: Baby's eyes open, can respond to light and sound
Common symptoms: Round ligament pain, backache, leg cramps, visible baby movements, Braxton Hicks contractions (practice contractions).
Third Trimester (Weeks 28-40+)
What's happening: Baby gains most of their weight during these final weeks (about 5 lbs), lungs mature, brain develops rapidly. You'll feel more uncomfortable as baby grows and takes up more space.
Key milestones:
- Week 32: Baby usually turns head-down in preparation for birth
- Week 36: Baby is considered "early term" (could be born safely, though 39-40 weeks is ideal)
- Week 37-40: Baby continues to gain weight (0.5 lb/week), lungs fully mature
- Week 39-40: Full term, ideal time for delivery
Common symptoms: Shortness of breath, frequent urination (baby pressing on bladder), swelling in feet/ankles, trouble sleeping, pelvic pressure, nesting instinct.
Important Pregnancy Dates and Appointments
Prenatal Appointment Schedule
- Weeks 4-28: Monthly appointments (every 4 weeks)
- Weeks 28-36: Bi-weekly appointments (every 2 weeks)
- Weeks 36-40+: Weekly appointments until delivery
Key Tests and Screenings
- Week 8-13: Dating ultrasound (confirm due date), first trimester screening (chromosomal abnormalities)
- Week 10-13: NIPT blood test option (non-invasive prenatal testing for genetic conditions, 99% accurate)
- Week 15-20: Quad screen blood test (spina bifida, Down syndrome screening)
- Week 18-22: Anatomy scan ultrasound (detailed check of baby's organs, growth, placenta position)
- Week 24-28: Glucose tolerance test (gestational diabetes screening, 10% of pregnancies)
- Week 35-37: Group B strep test (vaginal swab, 25% test positive, requires antibiotics during labor)
Frequently Asked Questions
How accurate is my due date?
Only 5% of babies arrive on their exact due date. The due date is best thought of as a "due month"—60% of babies are born within a week of the due date (before or after), and 90% are born within 2 weeks. First-time mothers are more likely to go past their due date (41 weeks average for first babies vs. 40 weeks for subsequent). Your due date may be adjusted after early ultrasound if baby measures significantly different from LMP-based calculation.
What if I don't remember my last period date?
If you don't know your last period date, your doctor will use an early ultrasound (ideally 8-13 weeks) to date your pregnancy. The ultrasound measures baby's crown-rump length (head to bottom), which is highly accurate for determining gestational age (within 3-5 days). After 13 weeks, ultrasounds become less accurate for dating because babies grow at different rates. If you have irregular cycles or were on birth control recently, ultrasound dating is more accurate than LMP anyway.
Can my due date change?
Your due date may be adjusted once, usually after an early ultrasound, if the measurement differs from your LMP-based date by more than 5-7 days. For example, if your LMP suggests you're 10 weeks but ultrasound measures baby at 8 weeks, your due date would be pushed back 2 weeks. Once established (usually by 13 weeks), the due date typically doesn't change again. Late pregnancy ultrasounds measure baby's size but shouldn't be used to change your due date because babies grow at different rates.
What happens if I go past my due date?
Going 1 week past your due date (41 weeks) is very common and normal, especially for first pregnancies. Your doctor will monitor you more closely with twice-weekly appointments checking baby's heart rate and amniotic fluid levels. Most doctors recommend inducing labor at 41-42 weeks to prevent complications like placenta aging, low amniotic fluid, or baby growing too large. Only 5-10% of pregnancies reach 42 weeks. If you prefer to wait, discuss the risks and benefits with your provider.
Are boy and girl pregnancies different lengths?
On average, boys are born slightly later than girls (1-2 days), but this difference is so small it's not clinically significant. What matters more: first babies average 1 week longer than subsequent babies (41 weeks vs 40 weeks), mothers over 35 tend to deliver slightly earlier, and maternal BMI can affect timing (underweight mothers tend to deliver earlier, overweight mothers slightly later). However, these are all statistical averages—individual pregnancies vary greatly.
Can I predict when labor will start?
No reliable way to predict exactly when labor will start. Signs that labor may be approaching in the next 1-3 weeks include: baby dropping lower (lightening), losing mucus plug (thick discharge, can happen weeks before labor), diarrhea or nausea, burst of energy (nesting instinct), and Braxton Hicks contractions becoming more frequent. True labor signs include: regular contractions that get closer together, stronger, and longer (5-1-1 rule: every 5 minutes, lasting 1 minute, for 1 hour), water breaking (clear or pinkish fluid), and bloody show (pink/brown discharge).
Medical Disclaimer: This pregnancy calculator provides estimates for educational purposes only and should not replace professional prenatal care. Every pregnancy is different. Your healthcare provider may adjust your due date based on ultrasounds and other factors. Always consult with your OB-GYN or midwife for personalized pregnancy care, advice on symptoms, and any concerns. Call your provider immediately if you experience severe abdominal pain, heavy bleeding, severe headache, vision changes, or decreased fetal movement.