Pregnancy Calculator
Welcome to the comprehensive pregnancy calculator designed to help expectant mothers and healthcare providers calculate due dates, track pregnancy progress, and understand fetal development. Calculate your baby's due date using multiple methods, determine your current pregnancy week and trimester, and access essential pregnancy information.
Calculate Your Pregnancy
Calculate by Last Menstrual Period (LMP)
Enter the first day of your last menstrual period to calculate your due date.
Calculate by Conception Date
Enter your known or estimated conception date.
Calculate by Known Due Date
Enter your due date to calculate current pregnancy week and trimester.
Calculate by Ultrasound Dating
Enter ultrasound measurements for accurate dating.
Your Pregnancy Information
Understanding Pregnancy Calculations
Naegele's Rule (LMP Method)
\[ \text{Due Date} = \text{LMP} + 280 \text{ days (40 weeks)} \]
Alternatively expressed as: LMP + 1 year - 3 months + 7 days
Conception Date Method
\[ \text{Due Date} = \text{Conception Date} + 266 \text{ days (38 weeks)} \]
Used when conception date is known (IVF, known ovulation, etc.)
Why 280 Days?
Pregnancy is medically calculated from the first day of your last menstrual period (LMP), not from conception. This is because the LMP is easier to identify than the exact conception date. The 280-day (40-week) calculation includes approximately 2 weeks before actual conception occurs, meaning fertilization typically happens around week 2 of your "pregnancy." This medical dating system is standard worldwide and provides consistent measurement regardless of individual cycle variations.
Calculation Methods Explained
Last Menstrual Period (LMP): The most common method used by healthcare providers. Naegele's Rule adds 280 days (40 weeks) to the first day of your last period. Accurate when you have regular 28-day cycles. May be adjusted for longer or shorter cycles.
Conception Date: Adds 266 days (38 weeks) from the known conception date. More accurate for women who know exactly when conception occurred (such as through IVF or fertility tracking). Conception typically occurs about 14 days after LMP in a 28-day cycle.
Ultrasound Dating: The most accurate method, especially when performed in the first trimester (weeks 8-13). Early ultrasounds measure crown-rump length (CRL) and are accurate to within 5-7 days. Dating ultrasounds become less accurate as pregnancy progresses.
Pregnancy Timeline by Trimester
First Trimester (Weeks 1-13)
Duration: Weeks 1-13 (approximately 3 months)
Key Developments:
- Weeks 1-4: Fertilization, implantation, and early embryonic development. Many women don't yet know they're pregnant
- Weeks 5-8: Baby's heart begins beating (week 6). Major organs start forming. Baby grows from 0.5cm to 2.5cm
- Weeks 9-13: All major organs formed. Baby's fingers and toes develop. By week 13, baby is about 7.5cm long
Common Symptoms:
- Missed period (first sign)
- Morning sickness (nausea and vomiting)—affects 70-80% of pregnant women
- Breast tenderness and enlargement
- Fatigue and exhaustion
- Frequent urination
- Food aversions or cravings
- Mood swings
- Light spotting or cramping (implantation bleeding)
Important Appointments:
- First prenatal visit (weeks 8-10): Medical history, physical exam, blood tests, urine tests
- Dating ultrasound (weeks 8-13): Confirm due date, detect heartbeat, check for multiples
- First trimester screening (weeks 11-14): Optional screening for chromosomal abnormalities
Second Trimester (Weeks 14-27)
Duration: Weeks 14-27 (approximately 3 months)
Key Developments:
- Weeks 14-17: Baby's skeleton hardens. Gender may be visible on ultrasound. Baby measures 10-15cm
- Weeks 18-22: You'll feel baby's movements (quickening). Baby develops sleep/wake patterns. Hearing develops
- Weeks 23-27: Baby's lungs develop. Eyes can open. Baby is about 35cm long and weighs 900g by week 27
Common Symptoms:
- Relief from morning sickness (for most women)
- Increased energy ("honeymoon period" of pregnancy)
- Growing belly becomes obvious
- Baby movements felt (18-25 weeks for first-time mothers)
- Back pain and round ligament pain
- Stretch marks may appear
- Pregnancy glow (increased blood flow to skin)
- Nasal congestion and nosebleeds
Important Appointments:
- Monthly prenatal visits (weeks 14-28)
- Anatomy scan (weeks 18-22): Detailed ultrasound checking baby's development and anatomy
- Glucose screening (weeks 24-28): Test for gestational diabetes
Third Trimester (Weeks 28-40+)
Duration: Weeks 28-40+ (approximately 3 months)
Key Developments:
- Weeks 28-32: Baby's eyes can open and close. Brain develops rapidly. Baby is about 42cm and weighs 1.8kg
- Weeks 33-36: Baby's bones harden (except skull). Lungs near maturity. Baby is about 47cm and weighs 2.7kg
- Weeks 37-40: Full term! Baby is ready for birth. Average size: 50cm long, 3.4kg weight
- Weeks 40+: Overdue pregnancy—closely monitored. Most babies born by week 42
Common Symptoms:
- Increased fatigue as belly grows larger
- Shortness of breath (baby pressing on diaphragm)
- Frequent urination (baby pressing on bladder)
- Braxton Hicks contractions (practice contractions)
- Swelling of ankles and feet
- Heartburn and indigestion
- Difficulty sleeping
- Back pain and pelvic pressure
- Nesting instinct
Important Appointments:
- Bi-weekly visits (weeks 28-36): More frequent monitoring
- Weekly visits (weeks 36-40+): Close monitoring as due date approaches
- Group B strep test (weeks 35-37)
- Non-stress tests if overdue (week 40+)
Important Pregnancy Milestones
| Week | Milestone | Significance |
|---|---|---|
| Week 4 | Positive Pregnancy Test | Missed period, hCG detectable in blood/urine |
| Week 6 | Fetal Heartbeat | Heart begins beating, visible on ultrasound |
| Week 12 | End of First Trimester | Miscarriage risk drops significantly |
| Week 18-20 | Quickening | Mother feels baby's movements |
| Week 20 | Anatomy Scan | Detailed ultrasound, gender often revealed |
| Week 24 | Viability | Baby could survive with intensive care if born |
| Week 28 | Third Trimester | Final growth phase, increased monitoring |
| Week 37 | Early Term | Baby is considered full term |
| Week 39-40 | Full Term | Optimal time for birth |
| Week 41+ | Post-term | Increased monitoring, possible induction |
Prenatal Care Essentials
Nutrition During Pregnancy
Proper nutrition supports your baby's growth and maintains your health throughout pregnancy.
- Folic Acid: 400-800 mcg daily before conception and throughout pregnancy. Prevents neural tube defects. Found in leafy greens, fortified cereals, and supplements
- Iron: 27 mg daily. Supports increased blood volume and prevents anemia. Found in red meat, beans, fortified cereals
- Calcium: 1,000 mg daily. Essential for baby's bone development. Found in dairy, fortified plant milk, leafy greens
- Protein: 75-100g daily. Critical for tissue growth. Found in meat, fish, eggs, legumes, dairy
- DHA (Omega-3): 200-300 mg daily. Supports brain and eye development. Found in fatty fish, walnuts, fortified eggs
- Hydration: 8-12 glasses of water daily. Supports increased blood volume and amniotic fluid
- Calories: Add 300-500 extra calories daily in second and third trimesters
Foods to Avoid
- Raw or undercooked meat, fish, and eggs (risk of bacteria and parasites)
- High-mercury fish (shark, swordfish, king mackerel, tilefish)
- Unpasteurized dairy products and soft cheeses (risk of listeria)
- Deli meats and hot dogs unless heated until steaming (listeria risk)
- Raw sprouts (bacteria risk)
- Alcohol (no safe amount during pregnancy)
- Excessive caffeine (limit to 200mg/day or less)
- Unwashed fruits and vegetables
Exercise and Activity
Regular exercise benefits both mother and baby, improving mood, reducing complications, and preparing the body for labor.
- Recommended: 30 minutes of moderate exercise most days of the week
- Safe Activities: Walking, swimming, prenatal yoga, stationary cycling, low-impact aerobics
- Avoid: Contact sports, activities with fall risk, scuba diving, hot yoga
- Warning Signs to Stop: Dizziness, chest pain, vaginal bleeding, decreased fetal movement, contractions
- Pelvic Floor Exercises: Kegel exercises strengthen pelvic floor for labor and postpartum recovery
Warning Signs - When to Call Your Doctor
Call Your Healthcare Provider Immediately If You Experience:
- Vaginal bleeding or spotting
- Severe abdominal pain or cramping
- Sudden swelling of face, hands, or feet
- Severe persistent headaches
- Vision changes (blurriness, flashing lights, spots)
- Fever above 38°C (100.4°F)
- Persistent vomiting or inability to keep food/liquids down
- Decreased fetal movement (after 28 weeks)
- Leaking fluid from vagina
- Regular contractions before 37 weeks
- Severe pain during urination
- Severe, sudden anxiety or feeling that something is wrong
Common Pregnancy Questions
How accurate is my due date?
Only about 5% of babies are born on their exact due date. A due date is an estimate representing when you'll be 40 weeks pregnant. Most babies are born between 38-42 weeks. First-trimester ultrasounds are most accurate for dating (within 5-7 days). LMP-based dating can be off if your cycles are irregular or you conceived during an unusual cycle.
When will I find out if I'm having a boy or girl?
Gender can typically be determined during the anatomy scan ultrasound at 18-20 weeks. Some earlier ultrasounds may detect gender around 14-16 weeks, though accuracy is lower. Non-invasive prenatal testing (NIPT) blood tests can determine gender as early as 10 weeks with high accuracy. Some parents choose to wait until birth for a surprise.
When will I feel my baby move?
First-time mothers typically feel movement (called "quickening") between 18-25 weeks. Mothers who've been pregnant before may feel movement earlier, around 16-18 weeks, because they recognize the sensation. Early movements feel like flutters, bubbles, or light tapping. By 24-28 weeks, movements become stronger and more regular. You should feel consistent movement patterns by the third trimester.
What is my due date if I have irregular periods?
LMP-based calculations are less accurate with irregular cycles. An early dating ultrasound (8-13 weeks) is the most reliable method for determining due date when cycles are irregular. Ultrasound measures the baby's size, which correlates closely with gestational age in early pregnancy. Your healthcare provider may adjust your due date based on ultrasound measurements rather than LMP.
Can my due date change?
Yes, due dates can be adjusted, typically only in the first trimester. If an early ultrasound shows the baby is significantly larger or smaller than expected based on LMP, your provider may revise your due date. After 20 weeks, due dates are rarely changed because babies grow at different rates. The most accurate due date comes from first-trimester ultrasound measurements combined with LMP data.
What happens if I go past my due date?
About 30% of pregnancies go past the due date. Post-term pregnancy (after 42 weeks) requires increased monitoring. Your provider will likely schedule more frequent appointments and tests (non-stress tests, biophysical profiles, amniotic fluid checks). Most providers recommend induction by 41-42 weeks to reduce risks of complications. Going a few days past your due date is completely normal and not concerning.
Why Choose RevisionTown Resources?
RevisionTown is committed to providing accurate, user-friendly calculators and educational resources across diverse topics. While we specialize in mathematics education for curricula like IB, AP, GCSE, and IGCSE, we also create practical tools for everyday needs, including health and wellness resources like this comprehensive pregnancy calculator.
Our pregnancy calculator offers multiple calculation methods to accommodate different situations—whether you know your last menstrual period, conception date, due date, or have ultrasound measurements. We provide detailed week-by-week timelines, trimester breakdowns, and essential pregnancy information to support expectant parents throughout their pregnancy journey.
Medical Disclaimer: This pregnancy calculator and information are for educational purposes only and should not replace professional medical care. Every pregnancy is unique, and complications can arise unexpectedly. Always consult with a licensed healthcare provider for pregnancy confirmation, prenatal care, and any health concerns. Due dates are estimates—only about 5% of babies are born on their exact due date. The information provided represents general pregnancy guidelines but may not apply to your specific situation. In case of emergency or concerning symptoms during pregnancy, contact your healthcare provider or go to the nearest emergency room immediately.
