Pregnancy Calculator
Calculate your due date, current pregnancy week, and trimester based on your last menstrual period or conception date.
Due Date Calculation
Naegele's Rule
Pregnancy Week
Understanding Pregnancy Dating
Pregnancy is typically calculated from the first day of your last menstrual period (LMP), not from conception. This means you're considered 'two weeks pregnant' at the time of conception, since ovulation typically occurs around day 14 of a 28-day cycle.
A full-term pregnancy is 40 weeks (280 days) from LMP, or 38 weeks from conception. This calculator uses Naegele's Rule, the standard method used by healthcare providers.
40 Week Journey
Full-term pregnancy spans 40 weeks from LMP, divided into three trimesters.
LMP Dating
Medical professionals date pregnancy from LMP, adding ~2 weeks before conception.
5% Accuracy
Only about 5% of babies arrive on their exact due date. Most arrive within 2 weeks.
Ultrasound Dating
Early ultrasounds can adjust due dates if measurements differ significantly.
The Three Trimesters
Pregnancy is divided into three trimesters, each with distinct developmental milestones:
First Trimester (Weeks 1-12)
Major organ development occurs. You may experience morning sickness, fatigue, and breast tenderness. By week 12, the baby is about 2 inches long and has all major organs forming.
Second Trimester (Weeks 13-27)
Often called the 'honeymoon period'—nausea typically subsides. You'll feel baby movements (quickening) around weeks 16-20. Baby grows to about 14 inches and develops fingerprints.
Third Trimester (Weeks 28-40)
Rapid weight gain for baby. You may experience Braxton Hicks contractions, back pain, and difficulty sleeping. Baby's lungs mature and they gain about half a pound per week.
Week-by-Week Development
Key developmental milestones throughout pregnancy:
| Week | Baby Size | Key Development | What You May Feel |
|---|---|---|---|
| 4 | Poppy seed | Implantation occurs | Missed period, fatigue |
| 8 | Raspberry | Heart beating, limbs forming | Morning sickness peaks |
| 12 | Lime | Reflexes develop, gender determined | Nausea may decrease |
| 16 | Avocado | Can hear sounds, eyes move | Energy returns, may feel movement |
| 20 | Banana | Halfway point, vernix forms | Definite movements felt |
| 24 | Corn | Lungs developing, responds to sounds | Visible kicks, Braxton Hicks may start |
| 28 | Eggplant | Eyes can open, brain developing rapidly | Shortness of breath, heartburn |
| 32 | Squash | Bones hardening, practicing breathing | Frequent urination, trouble sleeping |
| 36 | Honeydew | Baby 'drops' into pelvis | Easier breathing, pelvic pressure |
| 40 | Watermelon | Full term, ready for birth | Contractions, nesting instinct |
Due Date Accuracy
Due dates are estimates, not guarantees. Understanding the range helps set realistic expectations:
Statistical Reality
Only about 4-5% of babies arrive on their exact due date. About 80% are born within 2 weeks (before or after) of the due date.
Term Definitions
Early term: 37-38 weeks. Full term: 39-40 weeks. Late term: 41 weeks. Post-term: 42+ weeks. These distinctions matter for infant health outcomes.
First-Time Mothers
First pregnancies tend to go slightly longer than subsequent ones. First-time mothers are more likely to go past their due date.
Cycle Length Matters
Standard calculations assume a 28-day cycle. If your cycle is longer, your due date may be later. If shorter, it may be earlier.
Important Prenatal Milestones
Key appointments and tests during pregnancy:
First Prenatal Visit (Weeks 6-8)
Confirm pregnancy, establish due date, initial blood work, discuss medical history. May include early ultrasound to confirm viability.
First Trimester Screening (Weeks 11-14)
NT scan (nuchal translucency) and blood tests to screen for chromosomal conditions. Optional but recommended.
Anatomy Scan (Weeks 18-22)
Detailed ultrasound checking all major organs, spine, heart. Can often determine baby's sex if desired.
Glucose Test (Weeks 24-28)
Screening for gestational diabetes. You'll drink a glucose solution and have blood drawn to check sugar processing.
Group B Strep Test (Weeks 36-37)
Swab test for GBS bacteria. If positive, antibiotics during labor prevent transmission to baby.
Weekly Visits (Weeks 36-40)
More frequent monitoring as due date approaches. Checks for cervical changes, baby position, and signs of labor.
When to Call Your Provider
Contact your healthcare provider immediately if you experience:
Bleeding
Any vaginal bleeding, especially bright red or accompanied by cramping. Light spotting can be normal early on, but always report it.
Fluid Leaking
Gush or steady leak of fluid—could indicate ruptured membranes (water breaking) even before labor starts.
Severe Pain
Intense abdominal pain, especially if one-sided or constant. Could indicate ectopic pregnancy or placental issues.
High Fever
Temperature over 101°F (38.3°C) could indicate infection that may need treatment.
Decreased Movement
After week 28, if you notice significantly less baby movement than usual. Try drinking cold water and lying down first.
Frequently Asked Questions
Why is pregnancy counted from LMP, not conception?
LMP is a known, reliable date for most women, while exact conception date is often unknown. Ovulation and conception typically occur about 2 weeks after LMP, so the pregnancy dating system adds this time. This is why you're 'two weeks pregnant' at conception.
How accurate is my due date?
Due dates are estimates with a margin of error. Only about 5% of babies are born on their due date. Most healthy babies arrive between 38-42 weeks. Early ultrasound (before 12 weeks) can refine due dates to within 5-7 days.
What if my cycle isn't 28 days?
Standard calculations assume a 28-day cycle with ovulation on day 14. If your cycle is longer, you likely ovulated later, pushing your due date back. This calculator can adjust for different cycle lengths.
Can my due date change?
Yes. If ultrasound measurements differ significantly from LMP-based dating (more than a week in early pregnancy), your provider may adjust your due date to match ultrasound findings, which are more accurate.
What is full term pregnancy?
Full term is now defined as 39-40 weeks. Early term is 37-38 weeks, late term is 41 weeks, and post-term is 42+ weeks. Babies born at 39+ weeks have better outcomes than those born earlier.
How do I count pregnancy weeks?
Week 1 begins on the first day of your LMP. So at '4 weeks pregnant,' your last period started 4 weeks ago, and conception occurred about 2 weeks ago. You complete each week before moving to the next.
When will I start showing?
Most first-time mothers start showing between 12-16 weeks. Those who've been pregnant before may show earlier. Body type, muscle tone, and baby position all affect when your bump becomes visible.
When will I feel the baby move?
First-time mothers typically feel movement (quickening) between 18-22 weeks. Those who've been pregnant before may notice it as early as 14-16 weeks. Early movements feel like flutters or bubbles.
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