Pregnancy Calculator – Find Your Pregnancy Week, Trimester and Due Date Instantly

What Is a Pregnancy Calculator?

A Pregnancy Calculator is a free online tool that tells you how many weeks and days pregnant you are right now, which trimester you are in, your estimated due date, and key milestones in your pregnancy journey — all from a single input: the first day of your Last Menstrual Period (LMP).

Knowing exactly where you are in your pregnancy matters enormously. Healthcare appointments, prenatal screening tests, foetal development milestones, recommended supplements and safe activity guidelines all depend on gestational age. Most women learn they are pregnant several weeks after conception, making a reliable, instant pregnancy week calculator one of the first tools they reach for.

Our free Pregnancy Calculator uses Naegele’s Rule — the same calculation method used by obstetricians, midwives and gynaecologists worldwide — to determine your gestational age and estimated due date with clinical accuracy.

Use our free online Pregnancy Calculator above to find out exactly how many weeks pregnant you are.

How to Calculate How Many Weeks Pregnant You Are

Gestational age is counted from the first day of your Last Menstrual Period — not from the date of conception. This is a deliberate clinical convention used universally in obstetrics because the LMP date is known with certainty, while the exact date of conception is rarely known precisely.

The Gestational Age Formula

Gestational Age = Current Date − First Day of LMP

The result is expressed in weeks and days. A full-term pregnancy is approximately 40 weeks (280 days) from the first day of the LMP.

Naegele’s Rule — Estimated Due Date Formula

Estimated Due Date (EDD) = LMP + 280 days

Or equivalently:

EDD = LMP + 9 months + 7 days

This calculation assumes a regular 28-day menstrual cycle with ovulation occurring on Day 14. Variations in cycle length can shift the estimated due date by several days.

Example:

If the first day of your LMP was 1 January 2025:

The Three Trimesters of Pregnancy

A standard pregnancy is divided into three trimesters, each representing a distinct phase of foetal development and maternal physical change:

Understanding which trimester you are in helps you know what to expect from your body, which prenatal appointments and screenings are due, and what your baby is developing at that moment.

Week-by-Week Pregnancy Milestones

Each week of pregnancy brings measurable changes in foetal development and maternal experience. Here are key milestones across the 40-week journey:

These milestones help you contextualise your pregnancy week result from the calculator above and understand what is happening with your baby’s development right now.

Recommended Prenatal Appointments by Trimester

Knowing your gestational age allows you to anticipate upcoming prenatal appointments and ensure no critical screening is missed:

These timings are general guidelines. Your midwife or obstetrician will tailor the schedule to your specific circumstances, medical history and risk factors.

Why Knowing Your Pregnancy Week Matters

Understanding your exact gestational age from the moment you discover your pregnancy gives you practical advantages at every stage:

• Confirms your due date early — the earlier you know your EDD, the more effectively you can plan maternity leave, childcare, travel and family arrangements
• Guides prenatal supplement timing — folic acid is most critical in the first 12 weeks; knowing your gestational age tells you whether you are still in the window where it is most protective
• Determines safe screening windows — many prenatal tests such as the nuchal translucency scan and combined screening must be performed within a specific week range; knowing your week ensures you book them on time
• Tracks foetal growth against milestones — comparing your baby’s development to standard milestones by week helps you understand what is happening and what to expect at your next scan
• Informs safe exercise and activity levels — physical activity recommendations change by trimester; knowing your week helps you follow appropriate guidelines for your stage of pregnancy
• Helps identify symptoms that need attention — many symptoms are normal in one trimester but warrant medical attention in another; gestational age provides essential context for evaluating any symptom
• Prepares you for labour signs — understanding the difference between early-term (weeks 37–38), full-term (weeks 39–40) and post-term (weeks 41+) helps you know what to expect and when to contact your healthcare provider

Limitations of the Pregnancy Calculator

Our Pregnancy Calculator provides a highly useful gestational age and due date estimate, but several important limitations apply:

• Assumes a regular 28-day cycle — Naegele’s Rule was developed assuming ovulation occurs on Day 14 of a 28-day cycle. Women with shorter or longer cycles, or irregular cycles, may have a true EDD that differs from the calculator’s estimate by several days to a week or more
• Only 4–5% of babies are born on their due date — the EDD is a statistical midpoint of a normal distribution. Most babies are born within two weeks either side of the due date. A due date is best understood as the centre of a three-week birth window spanning weeks 37 to 42
• Ultrasound dating is more accurate in early pregnancy — a first-trimester ultrasound (ideally between weeks 8 and 13) measures the crown-rump length and typically provides a more accurate gestational age estimate than LMP-based calculation, particularly for women with irregular cycles
• The calculator cannot account for multiples — twin and triplet pregnancies are measured differently; average gestational length for twins is approximately 36 weeks, not 40
• Cycle irregularity shifts all downstream dates — if your LMP date is uncertain or your cycle length varies significantly, all dates produced by this calculator should be treated as approximate starting points to be refined by your healthcare provider

Who Should Use the Pregnancy Calculator?

Our free Pregnancy Calculator is useful for:

• Women who have just discovered they are pregnant — the most common first use of the tool; instantly answers “how many weeks am I?” and “when is my due date?”
• Women trying to conceive — tracking gestational age across cycles helps understand timing and supports use of our Ovulation Calculator and Conception Calculator
• Partners and family members — understanding the pregnancy week helps partners and family contextualise development milestones and upcoming appointments
• Women planning prenatal care — knowing your gestational age from the outset helps you schedule your booking appointment, first trimester screening and anatomy scan at the correct times
• Anyone who has had a positive pregnancy test — the first thing most people want to know after a positive test is how far along they are and when the baby is due

This calculator is a reference tool only and is not a substitute for:

• Confirmation of pregnancy by a qualified healthcare professional
• Ultrasound-based gestational age dating, which is more accurate
• Personalised prenatal care and midwifery or obstetric guidance

Frequently Asked Questions (FAQ)

How many weeks is a full-term pregnancy?

A full-term pregnancy is defined as 39 to 40 weeks and 6 days of gestation from the first day of the LMP. Early-term is defined as 37 to 38 weeks and 6 days. Late-term is 41 to 41 weeks and 6 days. Post-term is 42 weeks or more. The clinical definition of full-term was updated in 2013 by ACOG (American College of Obstetricians and Gynecologists) to reflect the importance of completing as much of the 39th week as safely possible for optimal neonatal outcomes.

Why is pregnancy measured from the LMP and not from conception?

Pregnancy is counted from the LMP because the exact date of conception — when the egg is fertilised — is almost never known with certainty, even with careful cycle tracking. The LMP date, by contrast, is known precisely by most women. Counting from the LMP adds approximately two weeks to the gestational age compared to counting from conception, which is why the embryo is technically about two weeks old at the start of “Week 3” of pregnancy.

What if I do not remember the exact first day of my LMP?

Use the closest date you can reliably recall and treat the calculator’s result as an approximate starting point. Your midwife or obstetrician will typically confirm or adjust your gestational age at your first appointment using a combination of your reported LMP date, physical examination and — most accurately — an early ultrasound. If you are more than a few days uncertain about your LMP, book an early ultrasound scan for more reliable dating.

How accurate is the due date produced by this calculator?

The EDD produced using Naegele’s Rule is accurate for women with regular 28-day cycles. Research shows that only 4–5% of women deliver on their precise estimated due date; approximately 80% deliver within two weeks either side of it. For women with irregular cycles or who are unsure of their LMP, first-trimester ultrasound dating is significantly more accurate and should be used to confirm or revise the EDD.

When should I have my first prenatal appointment?

Most healthcare providers recommend booking your first prenatal appointment — often called the booking appointment — at around 8 to 10 weeks of pregnancy. At this appointment, your midwife or doctor will confirm your pregnancy, take a detailed medical history, calculate or confirm your due date, arrange first-trimester blood tests, and organise your nuchal translucency scan for weeks 11–14. Do not wait until the second trimester to make this appointment.

What is the difference between gestational age and foetal age?

Gestational age is counted from the first day of the LMP — this is the standard clinical measure and what the calculator above uses. Foetal age (also called embryonic age or fertilisation age) is counted from the actual date of conception, which is typically about two weeks after the LMP. Gestational age is always approximately two weeks greater than foetal age for the same pregnancy. All clinical pregnancy guidelines, milestones and screening tests are based on gestational age.

Is it safe to use a pregnancy calculator if I have an irregular cycle?

Yes — the calculator is useful as a starting estimate even with irregular cycles. However, be aware that the result may be less accurate than for women with regular 28-day cycles. If your cycles are typically shorter than 28 days, your actual due date may be later than the calculator suggests; if your cycles are longer than 28 days, your due date may be earlier. An early ultrasound is strongly recommended to establish accurate gestational dating when cycle length is irregular.

Important Pregnancy Health Reminders by Trimester

First Trimester (Weeks 1–13)

• Begin taking folic acid (400 mcg daily) if you have not already — ideally before conception and continuing through the first 12 weeks to reduce the risk of neural tube defects
• Avoid alcohol, smoking and recreational drugs throughout pregnancy — the first trimester is the most critical period for foetal organ development
• Book your first prenatal appointment at around 8–10 weeks
• Common symptoms include nausea, fatigue, breast tenderness and frequent urination — these are normal but discuss any severe or unusual symptoms with your healthcare provider
• Schedule your nuchal translucency scan for weeks 11–14

Second Trimester (Weeks 14–27)

• The anatomy scan (20-week scan) is one of the most important appointments of your pregnancy — it checks the baby’s growth, organs and placental position; book it in advance
• Book your gestational diabetes screening (glucose tolerance test) for around weeks 24–28
• Energy typically improves during this trimester; gentle exercise such as walking, swimming and prenatal yoga is generally safe and beneficial
• Begin discussing your birth plan preferences with your midwife

Third Trimester (Weeks 28–40)

• Attend all scheduled third trimester appointments — your baby’s position, growth and wellbeing are monitored more closely as the due date approaches
• Be aware of the signs of preterm labour — regular, painful contractions before 37 weeks, sudden fluid loss, or significant changes in foetal movement all require immediate medical attention
• Foetal movement — from around 28 weeks onwards, your baby should move regularly. A sudden or sustained decrease in movements should be reported to your midwife or maternity unit the same day — do not wait until the next day
• Prepare your hospital bag from around week 34–36
• Discuss induction options with your healthcare provider if you reach 41 weeks without labour starting naturally

Final Thoughts

The Pregnancy Calculator is one of the most important and most used free online tools for expectant mothers. From the moment a pregnancy test shows positive, knowing how many weeks pregnant you are — and what your baby is developing right now — is a source of both practical guidance and profound connection to your pregnancy journey.

Use it together with the other free online pregnancy tools on CalcoraTools — the Due Date Calculator for a dedicated estimated due date, the Ovulation Calculator to understand fertile window timing, the Conception Calculator to estimate when conception occurred, the Pregnancy Weight Gain Calculator for guidance on healthy weight gain by trimester, and the Calorie Calculator to understand your increased nutritional needs during pregnancy.

Calculate your pregnancy week now — free, instant and no sign-up required.