Pregnancy Calculator
Due date and trimester timeline
How Due Dates Are Calculated
The estimated due date is calculated using Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). If your average cycle is longer or shorter than 28 days, the calculator adjusts accordingly. Only about 5% of babies are born on their exact due date, but most arrive within two weeks of this estimate.
Understanding Trimesters
Pregnancy is divided into three trimesters. The first trimester (weeks 1-12) is when major organ development occurs. The second trimester (weeks 13-26) is often considered the most comfortable period, when morning sickness typically subsides. The third trimester (weeks 27-40) involves rapid fetal growth and preparation for delivery.
Important Milestones
Key pregnancy milestones include: hearing the heartbeat at 6-7 weeks, feeling first movements (quickening) at 16-25 weeks, viability at about 24 weeks, and full term at 39-40 weeks. Your healthcare provider will schedule regular checkups and ultrasounds to monitor your pregnancy progress and baby's development.
Disclaimer
This calculator provides estimates for informational purposes only and should not replace professional medical advice. Always consult your healthcare provider for accurate dating and prenatal care guidance. Ultrasound dating, especially in the first trimester, is more accurate than LMP-based calculations.
Important health note
This calculator is for general information and self-checking only. It should not be used as a diagnosis, treatment plan, or replacement for advice from a qualified doctor, dietitian, trainer, or other healthcare professional. If you are pregnant, managing a medical condition, taking medication, recovering from illness, or making a major diet or exercise change, please speak with a professional before acting on the result.
Pregnancy Calculator: practical guide
The Pregnancy Calculator is built for people who want a fast answer without losing context. It keeps the calculation simple, shows the result clearly, and helps you understand what the number means before you use it in a real decision.
This calculator is designed to make a specific everyday calculation faster and clearer. It gives a structured result so you can compare options, check assumptions, or plan the next step with less manual work.
What is the best way to use the Pregnancy Calculator?
Enter the values carefully, review the units, and use the result as a reliable reference point. The Pregnancy Calculator is most useful when you compare scenarios or repeat the calculation with consistent inputs.
Is the Pregnancy Calculator accurate?
The calculator follows standard calculation logic, but accuracy depends on the values you enter and the assumptions behind the formula. For important health decisions, use it as guidance and verify the result with a trusted source.
How pregnancy due date is calculated
The standard method for calculating a pregnancy due date is Naegele's Rule, developed in the early 19th century. It assumes a 28-day menstrual cycle with ovulation on day 14 and calculates from the first day of the last menstrual period (LMP):
Due Date = First day of LMP + 280 days (40 weeks)
Or equivalently: Add 7 days to the LMP, subtract 3 months, and add 1 year.
Example: LMP = 1 January 2026. Add 7 days → 8 January. Subtract 3 months → 8 October. Add 1 year → 8 October 2026. Due date: 8 October 2026
Only approximately 5% of babies are born on their exact due date. The normal delivery window is from 37 to 42 weeks of gestation. Delivery before 37 weeks is considered preterm; after 42 weeks is post-term.
The three trimesters — what happens each trimester
First trimester (Weeks 1–12): The most critical developmental period. All major organs and body systems begin forming. The embryo becomes a fetus by week 10. Common symptoms include morning sickness (nausea and vomiting, often throughout the day), fatigue, breast tenderness, frequent urination, and food aversions. The risk of miscarriage is highest in the first trimester — approximately 80% of miscarriages occur before 12 weeks.
Second trimester (Weeks 13–26): Often called the "golden trimester" — nausea usually subsides, energy returns, and the pregnancy becomes visually apparent. The baby's movements (quickening) are typically felt for the first time between weeks 18–22. This trimester includes the anatomy scan ultrasound (usually at 18–20 weeks) which checks fetal development and can reveal the baby's sex.
Third trimester (Weeks 27–40): The baby grows rapidly — gaining approximately 50% of its birth weight in the final trimester. Common symptoms include back pain, heartburn, difficulty sleeping, and Braxton Hicks contractions. The baby typically assumes the head-down position (cephalic presentation) in preparation for delivery between weeks 32–36.
Key pregnancy milestones by week
- Week 6: Heartbeat detectable by transvaginal ultrasound
- Week 8: All major organs have begun to form; embryo is now called a fetus
- Week 12: End of first trimester; risk of miscarriage drops significantly; nuchal translucency scan
- Week 16–18: Amniocentesis window if indicated; baby's sex may be visible on ultrasound
- Week 20: Anatomy scan; halfway point
- Week 24: Viability milestone — babies born at 24+ weeks can survive with intensive care
- Week 28: Third trimester begins; glucose tolerance test for gestational diabetes
- Week 37: "Full term" — baby is considered ready for birth
- Week 40: Estimated due date
Gestational age vs fetal age
Gestational age counts from the first day of the last menstrual period — 2 weeks before conception typically occurs. Fetal age (also called embryonic age) counts from the moment of fertilisation. A baby at "40 weeks gestational age" is actually approximately 38 weeks of fetal development. Medical professionals universally use gestational age because the LMP date is usually known, while the exact conception date typically is not.
Prenatal care schedule (India — standard)
- First trimester: Booking appointment (8–12 weeks), blood tests, ultrasound for dating and nuchal translucency screening
- Second trimester: Anomaly scan at 18–20 weeks; regular weight, blood pressure, and urine checks every 4 weeks
- Third trimester: Visits every 2 weeks from week 28, then weekly from week 36; fetal position assessment; birth plan discussion
Frequently asked questions about pregnancy
How accurate is the due date calculation? Naegele's Rule assumes a 28-day cycle and ovulation on day 14. Women with irregular cycles or cycles longer/shorter than 28 days may have different ovulation timing. Early ultrasound (8–12 weeks) provides the most accurate due date estimate by measuring fetal size.
What if I don't know my last menstrual period date? An early ultrasound can estimate gestational age and calculate an approximate due date from fetal measurements. This is common for women with irregular periods or those who conceive shortly after stopping hormonal contraception.
What is a full-term pregnancy? Full term is defined as 39 weeks 0 days to 40 weeks 6 days. Early term is 37–38 weeks 6 days. Late term is 41 weeks 0 days to 41 weeks 6 days. Post-term is 42 weeks and beyond. Babies born at different term stages have different average outcomes.