Free Pregnancy Week Calculator
Track your pregnancy week by week with our free calculator. Enter your last menstrual period to see your current week, trimester, due date, and progress.
Based on Naegele's Rule (LMP + 280 days). This is an estimate — consult your healthcare provider for an accurate due date, especially if your cycles are irregular.
Formula
Estimated Due Date = Last Menstrual Period + 280 days (40 weeks) — known as Naegele's RuleUnderstanding Pregnancy Weeks and Due Dates
Pregnancy is typically measured in weeks starting from the first day of your last menstrual period (LMP), not from conception. This means that during the first two weeks of "pregnancy," you haven't actually conceived yet — this is simply how the medical community standardizes the timeline. A full-term pregnancy is 40 weeks, or 280 days from the LMP.
Our calculator uses Naegele's Rule, the same method most healthcare providers use for initial due date estimates. Named after German obstetrician Franz Naegele, this simple calculation adds 280 days to the first day of your last period. While straightforward, it assumes a regular 28-day cycle, so your provider may adjust the date based on ultrasound findings.
The Three Trimesters
Pregnancy is divided into three trimesters, each with distinct developmental milestones:
- First Trimester (Weeks 1-12): The most critical period for organ development. The brain, heart, and spinal cord begin forming. Common symptoms include morning sickness, fatigue, and breast tenderness. By week 12, all major organs and structures have formed.
- Second Trimester (Weeks 13-26): Often called the "golden period" as morning sickness typically subsides. The baby grows rapidly, and you may feel the first movements (quickening) around weeks 16-20. The anatomy scan ultrasound usually occurs around week 20.
- Third Trimester (Weeks 27-40): The baby gains most of its weight during this phase. The lungs mature, the brain develops rapidly, and the baby moves into a head-down position in preparation for birth. Braxton Hicks contractions may begin.
Important Milestones to Track
- Week 6-8: First heartbeat detectable via ultrasound
- Week 12: End of first trimester; risk of miscarriage decreases significantly
- Week 20: Halfway point; anatomy scan ultrasound
- Week 24: Viability milestone — baby has a chance of survival if born early
- Week 37: Considered early term; baby is nearly fully developed
- Week 40: Estimated due date
Remember that every pregnancy is unique. This calculator provides estimates based on standard timelines. Always work closely with your healthcare provider for personalized prenatal care.
Frequently Asked Questions
How is the due date 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). This assumes a 28-day cycle with ovulation on day 14. Only about 5% of babies are born on their exact due date — most arrive within two weeks before or after.
What if I don't know my last menstrual period date?
If you're unsure of your LMP, an ultrasound in the first trimester (before 14 weeks) can provide an accurate estimate of gestational age. Your healthcare provider can help determine your due date based on ultrasound measurements, particularly the crown-rump length of the embryo.
What happens during each trimester?
The first trimester (weeks 1-12) involves major organ development. The second trimester (weeks 13-26) is when the baby grows rapidly and you may feel movement. The third trimester (weeks 27-40) is the final growth phase where the baby gains weight and prepares for birth.
How accurate is a pregnancy due date?
Due dates are estimates. Only about 5% of babies arrive on the exact due date. A full-term pregnancy ranges from 37 to 42 weeks. First-time mothers tend to deliver slightly later than the due date. Factors like cycle length, maternal age, and prior pregnancies all influence timing.
What is gestational age vs. fetal age?
Gestational age is measured from the first day of your last menstrual period, which is about 2 weeks before conception actually occurs. Fetal age (also called embryonic age) is measured from the date of conception. Medical professionals use gestational age, which is why pregnancy is counted as 40 weeks even though the baby develops for approximately 38 weeks.