How many weeks pregnant am I? Calculate using period or due date

Quick take: You can figure out how many weeks pregnant you are by counting from the first day of your last menstrual period (LMP) or by counting backward from your estimated due date. Most calculators use a 40‑week (280‑day) pregnancy length, but individual variations are normal. If you’re unsure, try a reputable online calculator or ask your provider for a quick check.

It’s 2 a.m., you’re curled up on the couch, and a text from your partner says, “Hey, did we ever figure out how many weeks pregnant we are?” The thought of weeks and due dates can feel like math you never signed up for. You’ve probably heard terms like “gestational age,” “fetal age,” and “trimester” tossed around, and each one seems to mean something slightly different. The good news? The answer is actually straightforward once you know the two basic ways to count: from your last menstrual period (LMP) or from your estimated due date.

In this guide we’ll walk through exactly how to calculate your pregnancy weeks, explain why the numbers matter, and clear up common confusions like the difference between gestational age and fetal age. We’ll also point out factors that can shift the calculation, show you how to use a trusted online calculator, and give you a handy reference table that translates weeks into months and trimesters. By the end, you’ll feel confident about the number that pops up on your phone or in your prenatal chart.

Whether you’re just 8 weeks in or you’re approaching the 30‑week mark, the method you use—LMP or due date—will give you the same result if the dates are accurate. Let’s start by demystifying the terminology, then dive into the step‑by‑step math you can do at home or with a click.

Understanding pregnancy weeks and trimesters

Pregnancy is usually measured in weeks rather than months because weeks line up better with the timing of key developmental milestones and prenatal appointments. A full‑term pregnancy is about 40 weeks, counted from the first day of your last menstrual period (LMP). This is called the gestational age—the age of the pregnancy as measured by the calendar.

The term fetal age (or embryonic age) refers to how long the baby has actually been developing, which starts about two weeks after the LMP when conception typically occurs. In other words, fetal age is usually about two weeks less than gestational age. For most conversations with your obstetrician, the gestational age is the number they’ll use, because it aligns with standard screening schedules (e.g., the anatomy scan at 20 weeks gestational age).

Pregnancy is divided into three trimesters:

• First trimester: weeks 1 – 13 (gestational age). This is when the embryo implants and the placenta forms.
• Second trimester: weeks 14 – 27. The fetus grows rapidly, and many mothers feel a surge of energy.
• Third trimester: weeks 28 – 40. The baby gains most of its weight, and you’ll likely feel more physical discomfort.

The trimester breakdown helps providers schedule ultrasounds, blood tests, and other monitoring that are timed to the baby’s developmental stage. Knowing exactly which week you’re in ensures you get the right care at the right time.

In addition to the three‑trimester model, clinicians sometimes refer to the “early,” “mid,” and “late” phases of each trimester to fine‑tune counseling. For example, “early‑second‑trimester” (weeks 14‑18) is the window for chorionic villus sampling, while “late‑third‑trimester” (weeks 36‑40) is when labor‑onset counseling becomes a priority. Understanding where you fall on this spectrum can help you anticipate upcoming discussions with your care team.

How to calculate weeks from the first day of your last menstrual period

1. Identify the first day of your LMP. This is the first day of bleeding of your most recent menstrual cycle, not the day you ovulated or conceived. If your cycles are regular (about 28 days), the LMP is usually easy to recall.
2. Count the number of full weeks between that day and today. Start with the first day as week 0. Each Sunday (or any chosen day) marks the start of a new week. For example, if your LMP was March 1 and today is April 15, that’s 6 full weeks plus a few extra days, so you’re at 6 weeks + 2 days, often recorded as “6 weeks and 2 days.”
3. Add any extra days. Most clinicians round to the nearest whole week, but you can keep track of days for personal reference.
4. Confirm with a prenatal appointment. Your provider may adjust the calculation if an early ultrasound shows a different age.

Because pregnancy is measured from the LMP, the first two weeks are actually before conception. That’s why the gestational age starts at week 1 even though the embryo hasn’t formed yet. If you have irregular cycles or you’re unsure of the exact LMP, you can still estimate using the average length of your cycle (usually 28 days) and subtract or add a few days as needed.

Let’s try a quick example. Suppose your LMP was January 10 and today is June 5.

• January 10 → January 17 = week 1
• January 17 → January 24 = week 2
• …continue counting…
• May 29 → June 5 = week 22

That means you’re roughly 22 weeks pregnant (gestational age). If you wanted to know the fetal age, you’d subtract two weeks, giving a fetal age of about 20 weeks.

When you’re a few days past a full week, you’ll often see the notation “22 weeks + 4 days.” This level of detail can be useful for timing specific tests that are scheduled for a particular day of a week, such as the 24‑week anatomy scan, which is usually performed between 23 weeks 0 days and 25 weeks 6 days.

Using a due date to calculate pregnancy weeks

If you already know your estimated due date—perhaps because your provider gave you one after an early ultrasound—you can work backward to find out how many weeks pregnant you are. The standard formula assumes a 40‑week pregnancy, so you simply subtract the number of weeks between today’s date and the due date.

Here’s how:

1. Find your due date. This is typically calculated as 280 days (40 weeks) after the first day of your LMP, but it can be adjusted by an ultrasound measurement.
2. Count the weeks between today and the due date. If today is May 20 and your due date is October 15, you have about 21 weeks left until the due date.
3. Subtract that number from 40 weeks. 40 weeks − 21 weeks = 19 weeks gestational age.

For a quick mental check, you can use the “10‑month” rule: the due date is roughly 10 months after the LMP. If you’re 20 weeks pregnant, you’re about halfway through the pregnancy, which aligns with a due date roughly 20 weeks away.

Sometimes the due date feels like a distant, abstract number, especially early in the first trimester. Turning it into a weekly count helps you track milestones—like when you’ll hear the heartbeat at the 12‑week anatomy scan or when you’ll start feeling your baby move around week 20.

Remember that the due date is an estimate, not a guarantee. Only about 4 % of babies are born on their exact due date; most arrive within a two‑week window before or after. This is why clinicians talk about “estimated due date (EDD)” rather than “the day your baby will be born.”

Why accurate pregnancy week calculation matters

Knowing precisely how many weeks pregnant you are isn’t just a curiosity; it guides critical aspects of prenatal care:

• Screening schedules. Tests such as the first‑trimester combined screening (NT scan + blood work) are timed for weeks 11‑13 gestational age. A miscalculation could mean a missed window.
• Ultrasound timing. The anatomy scan at 18‑20 weeks is best performed when the fetus is big enough to see structures but small enough for the whole baby to fit in the screen.
• Medication dosing. Some prenatal vitamins and prescribed medications have dosage changes after certain weeks (e.g., iron supplementation after week 20).
• Travel and activity recommendations. Airlines often allow travel up to week 36 but may require a doctor’s note after week 28. Knowing your exact week helps you plan trips safely.

Guidelines from the American College of Obstetricians and Gynecologists (ACOG) and the National Health Service (NHS) both stress the importance of accurate dating for timing these interventions. Inaccurate dating can lead to premature or delayed screenings, which might affect early detection of conditions like chromosomal abnormalities or growth restriction.

Moreover, an accurate week count helps you set realistic expectations for symptoms. For example, “baby‑kick” sensations usually start between weeks 18‑22. If you’re at week 20 gestational age, you can anticipate that movement soon, whereas a miscalculated week could cause unnecessary worry.

Accurate dating also informs decisions about workplace accommodations, such as when to request a reduced‑hours schedule or when to discuss maternity leave. Many employers use gestational age to determine eligibility for certain benefits, so a reliable count can make those conversations smoother.

Factors that can affect pregnancy week calculation

While the LMP method is simple, several factors can shift the true gestational age:

• Irregular menstrual cycles. Women with cycles longer or shorter than 28 days may have a different ovulation day, which changes the conception date.
• Late ovulation. If you ovulate later in your cycle (e.g., day 20 instead of day 14), the standard LMP calculation overestimates the age by a few days.
• Assisted reproductive technologies (ART). IVF cycles often have a known embryo transfer date, which can be used to calculate a more precise fetal age.
• Early ultrasound dating. An ultrasound performed before 14 weeks can pinpoint gestational age within ±5 days, and many providers will adjust the due date based on that measurement.
• Misremembered LMP. Stress, illness, or simply forgetting can lead to an inaccurate start date. In such cases, a dating scan is the safest way to confirm.

Because of these variables, many clinicians will cross‑check the LMP calculation with an early scan. The World Health Organization (WHO) recommends confirming gestational age with a first‑trimester ultrasound whenever possible, especially for women with uncertain LMP dates.

When you’re unsure, it’s perfectly normal to ask your provider, “Can we re‑date my pregnancy based on the early scan?” They’ll explain any adjustments and what that means for your prenatal timeline.

Using an online pregnancy week calculator

Online calculators take the guesswork out of the arithmetic. You simply input either your LMP or your due date, and the tool instantly displays your gestational age, fetal age, current trimester, and a countdown to your due date.

Our own How Many Weeks Pregnant? calculator follows the same formulas used by clinicians and automatically adjusts for leap years and month lengths. It also shows you a visual timeline so you can see where you sit in the pregnancy journey.

When using any calculator, keep these tips in mind:

• Enter dates in the format the tool requests. Most calculators accept MM/DD/YYYY.
• Double‑check the result. If the weeks seem off, verify the LMP you entered matches medical records.
• Use it as a guide, not a diagnosis. If the calculator says you’re 22 weeks but your provider’s ultrasound says 20 weeks, trust the ultrasound.

These tools are especially handy for quick reference between appointments, for sharing with a partner, or for planning milestones like a gender reveal or a baby shower.

Tracking your pregnancy week with a journal or app

Beyond calculators, many parents find it helpful to keep a weekly pregnancy journal—either on paper or via a smartphone app. Recording the exact week lets you note symptoms, milestones, and emotions in a way that feels personal and organized.

Popular apps such as Ovia, What to Expect, and the NHS Pregnancy app sync with your LMP or due date and push weekly notifications that include fetal size comparisons, recommended nutrition, and upcoming appointments. According to the American College of Obstetricians and Gynecologists (ACOG), digital tracking can improve adherence to prenatal visits and promote early detection of warning signs.

If you prefer a paper journal, consider a printable weekly planner that includes space for “baby’s growth,” “my symptoms,” and “questions for my provider.” This can be a comforting ritual—writing down each week’s changes reminds you that pregnancy is a gradual, measurable journey, not a sudden leap.

When you notice a new symptom (e.g., sudden swelling or a change in fetal movement), jot it down with the week number. At your next visit, you’ll have a concise record to share, helping your clinician assess whether the change is typical for that gestational age.

How healthcare providers confirm gestational age

While self‑calculations are useful, the gold standard for confirming gestational age is an early ultrasound, typically performed between 8 and 14 weeks. The scan measures the crown‑rump length (CRL) of the embryo, which correlates tightly with gestational age.

According to the Royal College of Obstetricians and Gynaecologists (RCOG), a CRL measurement taken before 14 weeks can determine gestational age within ±5 days. After 14 weeks, the margin of error widens to about ±7 days because fetal growth rates become more variable.

If the ultrasound dating differs from your LMP calculation by more than a week, most providers will adopt the ultrasound date as the official gestational age. This adjustment can shift your estimated due date forward or backward, which in turn influences the timing of key screenings like the 20‑week anatomy scan or the glucose tolerance test at 24‑28 weeks.

In cases of multiple pregnancies (twins, triplets) or assisted reproductive technologies, the early scan is even more critical. Twins often have slightly different growth patterns, and IVF cycles provide a known embryo transfer date that can be cross‑checked with the CRL for precise dating.

Special considerations: multiples, IVF, and late dating

Women carrying twins or higher-order multiples typically deliver earlier than singletons, and their gestational age calculations may be adjusted for earlier onset of labor. The National Health Service (NHS) advises that twins are considered full term at 37 weeks, but many are born between 35‑36 weeks. Knowing the exact week helps you and your provider plan for potential preterm birth and neonatal care.

For those who conceive via IVF, the date of embryo transfer provides a clear starting point. Clinics often calculate “embryo age” based on the day the embryo was created (e.g., day 5 blastocyst) and then add two weeks to align with gestational age. This method can be more accurate than LMP dating, especially when menstrual cycles are irregular or suppressed.

If you discover your pregnancy later—say, in the second trimester—your provider will likely perform a detailed ultrasound to establish a reliable gestational age. This “late dating” scan focuses on biparietal diameter, femur length, and head circumference to estimate weeks. While the margin of error is larger than an early scan, it still provides enough precision to schedule necessary tests.

Regardless of how you arrived at your week count, keep the communication line open with your care team. Ask them to explain any adjustments and how they affect your prenatal plan.

Quick reference: weeks, months, and trimesters

Because many people think in months, here’s a handy table that translates weeks into months and identifies the trimester. It can be a quick way to answer “what month am I in?” without doing the math each time.

Use this table whenever you hear a week number and want to translate it into a more familiar month or trimester frame of reference.

From our medical team: The most reliable way to confirm how many weeks pregnant you are is an early ultrasound, especially if your menstrual history is unclear. However, for day‑to‑day tracking, the LMP method and reputable calculators are perfectly acceptable. If you notice a discrepancy between your personal calculation and what your provider tells you, bring it up at your next visit—small adjustments are common and help keep your care on schedule.

Myth vs. fact

Myth: You must be exactly 40 weeks pregnant to have a “full‑term” baby.

Fact: Full‑term births are defined as occurring between 37 weeks 0 days and 42 weeks 0 days gestational age. Babies born after 42 weeks are considered post‑term and may need closer monitoring.

Myth: If you know your due date, you don’t need to count weeks any other way.

Fact: The due date is an estimate based on the LMP or early ultrasound. Counting weeks from the LMP helps you understand where you are on the timeline and prepares you for appointments that are scheduled by gestational week.

Myth: Irregular periods make pregnancy dating impossible.

Fact: While irregular cycles can introduce uncertainty, an early ultrasound can accurately determine gestational age, and most clinicians will adjust the due date accordingly.

Key takeaways

• Gestational age (weeks from LMP) is the standard way to say “how many weeks pregnant am I.”
• Count from the first day of your last menstrual period, or subtract weeks from your due date to find the same number.
• Accurate week calculation guides screening tests, medication timing, and travel recommendations.
• Irregular cycles, IVF, or early‑ultrasound findings can shift the calculated age; always discuss adjustments with your provider.
• Trusted online calculators, like our How Many Weeks Pregnant? tool, give instant results and visual timelines.
• If anything feels off—missed periods, unexpected symptoms, or a mismatch with your provider’s numbers—schedule a quick check‑up.

Frequently asked questions

How do I calculate my due date?

The most common method is the “Naegle’s Rule”: add 280 days (40 weeks) to the first day of your last menstrual period. Adjust for cycle length if it’s not 28 days, and confirm with an early ultrasound if available.

What week of pregnancy am I in?

To find out, count the full weeks from the first day of your LMP to today, or use a due‑date calculator to see how many weeks remain until 40 weeks. Your provider will record this as your gestational age.

Can I calculate my pregnancy weeks from conception?

Yes, but you’ll get the fetal age, which is about two weeks less than gestational age. If you know the exact date of conception (e.g., via IVF), add the days to that date and subtract from today’s date to get fetal weeks, then add two weeks to express it as gestational age.

How accurate are pregnancy week calculators?

Calculators are accurate when you enter correct dates. They use the same 280‑day formula that clinicians use. However, variations in cycle length, ovulation timing, or early‑ultrasound measurements can cause a few days’ difference, which is normal.

What is the difference between gestational age and fetal age?

Gestational age starts from the first day of your LMP and is used for clinical scheduling. Fetal age counts from conception, usually about two weeks later. Most medical records list gestational age.

How does my last menstrual period affect my pregnancy week calculation?

The LMP provides the anchor point for counting weeks. If your cycles are regular, the LMP gives a reliable estimate. If cycles are irregular, the LMP may over‑ or underestimate the true gestational age, which is why an early ultrasound is helpful.

Can I trust my partner’s memory of my LMP?

Many couples keep a shared calendar or app to track periods, which can improve accuracy. If you’re unsure, compare your recollection with any pharmacy records, a home‑pregnancy test date, or ask your provider for an early‑ultrasound confirmation.

What if my period fell on a leap year?

Leap years add an extra day (February 29). Most calculators automatically account for this, but if you’re doing the math by hand, remember to include the extra day when counting weeks between dates that span February of a leap year.

When to call your doctor

If you notice any of the following, reach out to your obstetrician or midwife right away: sudden severe abdominal pain, heavy bleeding, loss of fetal movement after week 28, fever over 100.4 °F (38 °C), or any new, concerning symptoms. This article is for general information only and does not replace personalized medical advice.

References

1. American College of Obstetricians and Gynecologists (ACOG). “Ultrasound Guidance for Pregnancy Dating.” Clinical Guidance, 2023.
2. National Health Service (NHS). “How to calculate your due date.” Patient Information, 2022.
3. World Health Organization (WHO). “Maternal, Newborn, Child and Adolescent Health: Guidelines for Antenatal Care.” 2021.
4. Mayo Clinic. “Pregnancy week by week.” Health Library, 2024.
5. Centers for Disease Control and Prevention (CDC). “Prenatal Care.” 2023.
6. Royal College of Obstetricians and Gynaecologists (RCOG). “Dating and monitoring of pregnancy.” Clinical Practice Guidelines, 2022.
7. Food and Drug Administration (FDA). “Guidance for Industry: Use of Pregnancy Exposure Registries.” 2020.
8. American College of Obstetricians and Gynecologists (ACOG). “Use of Mobile Apps in Obstetric Care.” Committee Opinion, 2021.