Pregnancy Due Date Calculator: Understanding Your Timeline
Learn how pregnancy due dates are calculated, what to expect in each trimester, and how to track your pregnancy week by week from conception to delivery.
How Due Dates Are Calculated
Pregnancy due dates are estimated using Naegele's Rule, a formula developed in the early 1800s that remains the standard today. The calculation assumes a 28-day menstrual cycle with ovulation occurring on day 14.
The formula adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). While simple, this method is remarkably accurate - approximately 80% of babies are born within 10 days of their due date, though only about 5% arrive exactly on the predicted date.
Ways to Calculate Your Due Date
Last Menstrual Period (LMP) Method
The most common calculation: add 280 days to the first day of your last period. Use our due date calculator for instant results.
Conception Date Method
If you know when conception occurred, add 266 days (38 weeks). This is more accurate but requires knowing the exact date of conception.
Ultrasound Dating
First-trimester ultrasounds (before 13 weeks) measure crown-rump length to estimate gestational age within 5-7 days. This method is most accurate early in pregnancy.
IVF Dating
For IVF pregnancies, the date is calculated from embryo transfer. A 5-day embryo transfer date plus 261 days equals the due date.
First Trimester: Weeks 1-12
The first trimester begins on the first day of your last period (before conception occurs) and continues through week 12.
Weeks 1-4
Conception typically occurs around week 2. By week 4, the fertilized egg implants in the uterine wall. Most women do not yet know they are pregnant.
Weeks 5-8
The embryo develops rapidly. The heart begins beating around week 6. Morning sickness often begins during this period. First prenatal appointment usually scheduled around week 8.
Weeks 9-12
The embryo becomes a fetus at week 9. All major organs are forming. By week 12, the risk of miscarriage decreases significantly. First-trimester screening (NT scan and blood tests) typically occurs weeks 11-13.
Common First Trimester Symptoms
- Fatigue and increased need for sleep
- Morning sickness (can occur any time of day)
- Breast tenderness and changes
- Frequent urination
- Food aversions and cravings
- Mood changes
Second Trimester: Weeks 13-27
Often called the "golden trimester" as many first-trimester symptoms subside and energy returns.
Weeks 13-16
Energy levels typically improve. The baby begins making facial expressions. Gender may be visible on ultrasound by week 16.
Weeks 17-20
Quickening - feeling the baby's first movements - typically occurs between weeks 18-22 (earlier for subsequent pregnancies). The anatomy scan (detailed ultrasound) usually occurs around week 20.
Weeks 21-27
The baby's hearing develops, and they may respond to sounds. Weight gain accelerates. The baby is viable outside the womb by approximately week 24, though outcomes improve significantly each additional week.
Common Second Trimester Symptoms
- Visible baby bump
- Increased appetite
- Skin changes (linea nigra, stretch marks)
- Backaches as the uterus grows
- Nasal congestion
- Leg cramps
Third Trimester: Weeks 28-40
The final stretch as the baby gains weight rapidly and prepares for birth.
Weeks 28-32
The baby's lungs are maturing. Brain development accelerates. Most babies are head-down by week 32, though breech position may change later.
Weeks 33-36
The baby gains about half a pound per week. Bones are hardening (except skull bones, which remain flexible for birth). Group B strep test typically performed week 35-37.
Weeks 37-40
The baby is considered full-term at 39 weeks. Lightening may occur as the baby drops lower into the pelvis. Watch for signs of labor: contractions, water breaking, bloody show.
Common Third Trimester Symptoms
- Shortness of breath (as uterus presses on diaphragm)
- Difficulty sleeping
- Braxton Hicks contractions
- Swelling in feet and ankles
- Heartburn
- Frequent urination returns
- Lower back pain
Understanding Due Date Accuracy
Due dates are estimates, not predictions. Here's what research tells us:
- Only about 5% of babies arrive on their exact due date
- Full-term pregnancy ranges from 39 weeks 0 days to 40 weeks 6 days
- First-time mothers often deliver a few days after the due date
- Second and subsequent babies often come earlier
- Post-term is defined as 42 weeks or later
Tracking Your Pregnancy
Use our due date calculator to determine your estimated due date and track your pregnancy progress. The calculator shows:
- Your estimated due date
- Current gestational age in weeks and days
- Which trimester you are in
- Key milestone dates
Conclusion
While due dates provide a helpful framework for planning, every pregnancy progresses uniquely. Use the estimated date as a guide while remaining flexible. Regular prenatal care allows your healthcare provider to monitor your specific pregnancy and adjust expectations as needed.
Track your pregnancy journey with our due date calculator, monitor weight changes appropriately using our BMI calculator, and celebrate each milestone on the path to meeting your little one.
CalcOnce Editorial Team
The CalcOnce team creates comprehensive guides and free calculators to help you make better decisions. Our content is researched thoroughly and updated regularly to ensure accuracy.