Pregnancy Week Calculator
Calculate pregnancy week quickly with our pregnancy tool. Get results based on evidence-based formulas with clear explanations.
Medical disclaimer: This calculator is provided for educational and informational purposes only and does not constitute medical advice, diagnosis, or treatment. Results are general estimates and may not reflect your individual circumstances. Always consult a qualified healthcare professional before making decisions about your health.
Pregnancy Week Calculator
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Formula: Due Date = LMP + 280 days (Naegele's Rule)
Worked example — Week 9, Day 2 | 1st Trimester | Due: October 22, 2026 | 215 days remaining
Formula
Due Date = LMP + 280 days (Naegele's Rule)
The estimated due date is calculated by adding 280 days (40 weeks) to the first day of the last menstrual period (LMP). Current gestational week = days since LMP / 7. Trimester: 1st = weeks 1-12, 2nd = weeks 13-26, 3rd = weeks 27-40. If due date is known, LMP is back-calculated as due date minus 280 days.
Worked Examples
Example 1: LMP Date Input — Early Pregnancy
Problem:Last menstrual period was January 15, 2026. What is the current pregnancy status? (Assume today is March 21, 2026.)
Solution:LMP: January 15, 2026 Due date (LMP + 280 days): October 22, 2026 Days since LMP: 65 days Current week: 65 / 7 = Week 9, Day 2 Trimester: 1st (weeks 1-12) Days remaining: 280 - 65 = 215 days Baby size comparison: Cherry
Result:Week 9, Day 2 | 1st Trimester | Due: October 22, 2026 | 215 days remaining
Example 2: Due Date Input — Late Pregnancy
Problem:Due date is April 15, 2026. What is the current status? (Assume today is March 21, 2026.)
Solution:Due date: April 15, 2026 LMP (due date - 280 days): July 9, 2025 Days since LMP: 255 days Current week: 255 / 7 = 36 remainder 3, so Week 36, Day 3 Trimester: 3rd (weeks 27-40) Days remaining: 280 - 255 = 25 days Baby size comparison: Romaine lettuce
Result:Week 36, Day 3 | 3rd Trimester | Due: April 15, 2026 | 25 days remaining
Frequently Asked Questions
How is pregnancy due date calculated?
Pregnancy due date is most commonly calculated using Naegele's rule, which adds 280 days (40 weeks) to the first day of the last menstrual period (LMP). This assumes a 28-day menstrual cycle with ovulation occurring on day 14. The formula was developed by German obstetrician Franz Naegele in the early 1800s and remains the standard method used by healthcare providers today. However, only about 4% of babies are born on their exact due date. Most babies arrive within two weeks before or after the estimated date. Ultrasound dating in the first trimester provides more accurate dating, especially for women with irregular cycles, and may adjust the estimated due date by several days.
What are the three trimesters of pregnancy?
Pregnancy is divided into three trimesters, each approximately 13 weeks long. The first trimester (weeks 1-12) is the period of rapid organ development when all major body systems form. This is when morning sickness is most common and the risk of miscarriage is highest. The second trimester (weeks 13-26) is often called the golden period because many uncomfortable symptoms subside, energy increases, and the baby bump becomes visible. Fetal movements (quickening) are typically first felt around week 20. The third trimester (weeks 27-40) involves rapid fetal growth and brain development. The baby gains most of its weight during this period, and the mother may experience increased fatigue, back pain, and Braxton Hicks contractions.
What is the difference between gestational age and fetal age?
Gestational age counts pregnancy from the first day of the last menstrual period (LMP), even though conception typically occurs about two weeks later during ovulation. This means gestational age is approximately two weeks ahead of the actual fetal age (also called conceptional age). When your doctor says you are 8 weeks pregnant, the fetus is actually about 6 weeks old. Gestational age is the standard used in clinical practice because LMP date is usually known with more certainty than the exact date of conception. All pregnancy milestones, growth charts, and due date calculations reference gestational age. Understanding this distinction helps explain why pregnancy is described as 40 weeks even though the fetus develops for approximately 38 weeks.
When should I first see a doctor during pregnancy?
Most healthcare providers recommend scheduling your first prenatal visit between weeks 6 and 8 of pregnancy, counted from the first day of your last menstrual period. At this visit, the provider confirms the pregnancy with blood work and potentially an early ultrasound, establishes the due date, reviews medical history, orders baseline lab work, and discusses prenatal vitamins. However, you should contact your provider immediately if you experience severe abdominal pain, heavy bleeding, high fever, or severe vomiting causing dehydration. After the initial visit, prenatal appointments are typically monthly until week 28, every two weeks until week 36, then weekly until delivery. More frequent monitoring may be recommended for high-risk pregnancies.
Is it normal to deliver before or after the due date?
Yes, it is completely normal and very common. Only about 4-5% of babies are born on their exact estimated due date. Approximately 80% of babies are born between weeks 37 and 42, which is considered the normal range. First-time mothers tend to deliver slightly later than their due date on average. A pregnancy is considered early term at 37-38 weeks, full term at 39-40 weeks, late term at 41 weeks, and post-term at 42 weeks or beyond. Most providers will discuss induction of labor if pregnancy extends beyond 41-42 weeks due to increased risks associated with post-term pregnancy. Factors influencing delivery timing include genetics, the number of previous pregnancies, maternal age, and individual fetal development patterns.
Why does Naegele's rule assume a 28-day cycle, and what if mine is longer?
Naegele's rule adds 280 days to the first day of the last menstrual period because it assumes ovulation on cycle day 14, giving roughly 266 days of actual embryonic development. If your cycles run consistently longer than 28 days, ovulation happens later, so the LMP-based due date is shifted early by roughly the number of days your cycle exceeds 28. Some clinicians apply that correction manually; more often an early ultrasound settles the dating instead. Discuss any cycle irregularity with your provider so the estimate is not misread.
How does a first-trimester ultrasound redate a pregnancy?
Early ultrasound dating measures crown-rump length, the distance from the top of the head to the bottom of the torso. In the first trimester embryos grow at a very uniform rate, so this measurement maps tightly onto gestational age. If the ultrasound-derived age differs from the LMP-derived age by more than a small margin, providers usually adopt the ultrasound date and recalculate the due date from it. Later scans use head, abdomen and femur measurements, which vary far more between fetuses and are therefore weaker for dating.
What does a gestational age written as 'weeks plus days' mean?
Clinical notes express gestational age as completed weeks plus extra days, such as 32+4. That means 32 full weeks have elapsed since the first day of the last menstrual period plus four additional days, or 228 days total. Pregnancy Week Calculator uses the same convention: it divides days since LMP by seven, takes the whole number as the week and the remainder as the day. The notation matters because growth charts, screening windows and viability thresholds are all defined to the day, not the week.
How do Braxton Hicks contractions differ from true labor contractions?
Braxton Hicks are irregular, painless or mildly uncomfortable tightenings of the uterine muscle that can appear from the second trimester onward and become more noticeable in the third. They do not follow a pattern, do not intensify, and typically settle with a change of position, hydration or rest. True labor contractions come at shortening intervals, grow stronger and longer, persist through movement, and progressively efface and dilate the cervix. Regular painful contractions before 37 weeks should be reported to your provider promptly.
What does the 'baby size' comparison for each week actually represent?
The fruit and vegetable comparisons are a rough visual proxy for average fetal length at that gestational week, not a measurement of your own pregnancy. Early on they track crown-rump length; from roughly the second trimester they track crown-heel length, which is why the apparent jump in size seems abrupt around the midpoint. Real fetuses vary substantially in size at any given week, and only biometry measured on ultrasound and plotted against a growth chart says anything meaningful about growth.
References
Reviewed for accuracy by Rahul Singh, Health & Wellness Specialist · Editorial policy
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