Pregnancy Calculator & Due Date
Calculate your pregnancy due date, conception date, and track your journey week by week. Get trimester breakdowns and important milestone dates with our free pregnancy calculator.
The first day of your last menstrual period
Typical range: 21-35 days (average is 28)
How to Use the Pregnancy Calculator
Step 1: Choose Calculation Method
Select how you want to calculate: Last Menstrual Period (most common), Conception Date (if known), or Known Due Date (from doctor/ultrasound).
Step 2: Enter Your Date
For LMP: Enter the first day of your last period and average cycle length. For conception: Enter the estimated conception or ovulation date. For due date: Enter the date provided by your healthcare provider.
Step 3: View Results
Get your estimated due date, current gestational age, trimester information, days remaining, and important milestone dates throughout your pregnancy.
Step 4: Track Your Progress
Use the calculator regularly to see updated week-by-week progress. Copy your results to track milestones and share with your healthcare provider.
Step 5: Plan Important Appointments
Reference the milestone dates to schedule prenatal visits, ultrasounds, glucose screening, and other important tests at the right time.
Step 6: Consult Healthcare Provider
Always verify your due date with your doctor or midwife. Early ultrasounds provide the most accurate dating and should be used to adjust calculator estimates.
Understanding Pregnancy Dating
Naegele's Rule
The standard method for calculating due dates: LMP + 280 days (40 weeks). Assumes a 28-day cycle with ovulation on day 14. Formula: (Last Period Date) + 7 days - 3 months + 1 year.
Gestational Age vs Fetal Age
Gestational age: Counted from first day of last period (what doctors use). Fetal age: Counted from conception (~2 weeks later). When someone says "12 weeks pregnant," they mean 12 weeks gestational age.
Ultrasound Dating
First-trimester ultrasounds (8-13 weeks) measure crown-rump length (CRL) and are accurate within ±3-5 days. This is more accurate than LMP dating, especially for women with irregular cycles.
Cycle Length Adjustments
For cycles longer or shorter than 28 days, adjustments are needed. If your cycle is 32 days, add 4 days to the standard due date. If 24 days, subtract 4 days. Ovulation timing affects conception date.
Trimester-by-Trimester Guide
First Trimester (Weeks 1-13)
- • Major organ development begins
- • Morning sickness and fatigue common
- • Heartbeat detectable by week 6
- • First prenatal visit (weeks 8-12)
- • Risk of miscarriage highest in first 12 weeks
- • Baby is size of a peach by week 13
Second Trimester (Weeks 14-27)
- • Energy returns, "pregnancy glow"
- • Baby movement felt (weeks 16-25)
- • Anatomy scan and gender reveal (weeks 18-22)
- • Glucose screening test (weeks 24-28)
- • Belly becomes visibly rounded
- • Baby is size of a cauliflower by week 27
Third Trimester (Weeks 28-40)
- • Rapid weight gain for baby
- • Braxton Hicks contractions may start
- • Group B Strep test (weeks 35-37)
- • Baby drops into birth position (weeks 36-38)
- • Full term at 37 weeks, optimal at 39-40
- • Baby is 6-9 lbs at birth
Frequently Asked Questions
How accurate is the pregnancy due date calculator?
Pregnancy calculators provide estimates based on standard 40-week gestations. Only about 5% of babies arrive on their exact due date. Most deliver within 2 weeks before or after. Ultrasound dating (especially in the first trimester) is more accurate than LMP-based calculations.
How is pregnancy calculated from last menstrual period?
Pregnancy is calculated using Naegele's Rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a 28-day cycle with ovulation on day 14. For different cycle lengths, adjustments are made. Gestational age is counted from LMP, not conception.
What's the difference between gestational age and fetal age?
Gestational age (pregnancy age) is measured from the first day of your last period, while fetal age (conceptional age) is measured from actual conception. Gestational age is typically 2 weeks longer than fetal age since conception occurs around day 14 of the menstrual cycle.
When does each trimester start and end?
First trimester: Weeks 1-13, second trimester: Weeks 14-27, third trimester: Weeks 28-40. The first trimester involves major organ development, the second is often the most comfortable with visible growth, and the third prepares mother and baby for delivery.
Can my due date change during pregnancy?
Yes, your due date may be adjusted based on early ultrasound measurements, especially if done between 8-13 weeks. First-trimester ultrasounds are very accurate (±3-5 days) and may reveal a different due date than LMP calculations, particularly if ovulation was irregular.
How do I calculate my conception date?
Conception typically occurs about 14 days after the start of your last period (ovulation day for a 28-day cycle). If you know your due date, subtract 266 days (38 weeks) to estimate conception. For irregular cycles, ovulation predictor kits or ultrasound dating provide better estimates.
What is considered full term pregnancy?
Full term is 39-40 weeks. Early term is 37-38 weeks, full term is 39-40 weeks, late term is 41 weeks, and post-term is 42+ weeks. Babies born at 37 weeks or later are considered term, but 39-40 weeks allows for optimal development.
How long is pregnancy in days and weeks?
Pregnancy lasts approximately 280 days or 40 weeks from the first day of your last menstrual period. From conception, it's about 266 days or 38 weeks. However, normal pregnancy can range from 37-42 weeks. Most women deliver between weeks 38-42.
When will I have my first ultrasound?
The first ultrasound is typically done between 8-12 weeks (dating scan) or 18-22 weeks (anatomy scan). Early ultrasounds confirm pregnancy location, viability, and accurate dating. The 20-week anatomy scan checks fetal development and can reveal gender.
What if I don't know my last period date?
If you don't remember your LMP, use the conception date if known, or wait for an early ultrasound (8-13 weeks) which can accurately date your pregnancy. Ultrasound measurements of crown-rump length (CRL) are very precise in the first trimester.