Pregnancy Calculator

Pregnancy Calculator: Weeks, Trimesters & Fetal Development

Introduction: Why a Due Date Matters

Finding out you’re pregnant brings one immediate question: when is my baby due?

A pregnancy calculator answers that question in seconds. It takes a starting date — usually the first day of your last period — and turns it into an estimated due date (EDD), your current week of pregnancy, and your trimester.

This calculator is built for anyone in early or ongoing pregnancy: people who just got a positive test, people preparing for a first prenatal visit, and people who already have an ultrasound date and want a more precise estimate. It also works for people using conception tracking or IVF, since those paths use different math than a standard menstrual-cycle estimate.

Knowing your due date matters for more than curiosity. It sets the schedule for prenatal screening, anatomy scans, and the point at which your pregnancy is considered full-term. It also gives every other pregnancy milestone — fetal development, trimester transitions, appointment timing — a fixed reference point.

In this guide, you’ll find:

• The four due-date calculation methods and their exact formulas
• Step-by-step instructions for using the calculator
• Three fully worked examples
• A fetal-development breakdown by trimester, including a size-by-week table
• A method comparison table, common mistakes, and risk factors
• A glossary and an expanded FAQ

What Is a Pregnancy Calculator?

A pregnancy calculator is a tool that estimates your due date and current gestational age from a known reference date. You may also see it called a due date estimator or gestation calculator — they all use the same underlying formulas.

Every method the calculator supports relies on the same core idea: pregnancy is measured in gestational age, which is counted from the first day of your last menstrual period (LMP), not from the actual day of conception. This is why gestational age and fetal age are not the same number — gestational age typically runs about two weeks ahead of fetal (conceptional) age.

How to Use the Pregnancy Calculator

The calculator accepts one primary input plus several optional fields that improve accuracy.

1. Enter your Last Menstrual Period (LMP) date. This is the first day of your most recent period. It’s required unless you’re entering a conception date, IVF transfer date, or ultrasound date instead.
2. Enter your average cycle length. The default is 28 days. If your cycles run longer or shorter, adjust this field — it shifts the assumed ovulation day and refines your estimate.
3. Enter a conception date (optional). If you know it precisely — for example, from ovulation tracking — this overrides the LMP-based calculation.
4. Enter an IVF transfer date and embryo age (optional). Choose Day 3 or Day 5 transfer. This overrides both LMP and conception-date calculations because embryo age is known with clinical precision.
5. Enter an ultrasound date and gestational age at scan (optional). A first-trimester dating ultrasound is the most accurate input available and takes priority over every other method, per standard clinical convention.

The calculator then returns your estimated due date, current gestational age, current trimester, and key milestone windows (anatomy scan, viability, full-term). Results appear immediately without a page reload, and every result carries a visible disclaimer: this is an estimate, not a guaranteed birth date.

The Four Methods for Calculating a Due Date

Pregnancy due dates can be calculated four common ways, depending on what information you have available. Each method uses a different starting point, but all of them aim to estimate the same target: a 40-week gestational timeline.

Method 1: Last Menstrual Period (Naegele’s Rule)

This is the most widely used method because it requires only one piece of information most people already know.

Formula: EDD = LMP + 280 days (adjusted for cycle length)

This calculation, known as Naegele’s Rule, is the dating convention endorsed by the American College of Obstetricians and Gynecologists (ACOG) for initial due-date estimation. The rule assumes a 28-day cycle with ovulation occurring on day 14 — the midpoint of a typical cycle, based on the average time it takes the ovary to release an egg after the start of a period.

If your cycle isn’t 28 days, the calculator adjusts the math: add or subtract the difference between your actual cycle length and 28 days. A 32-day cycle, for instance, shifts ovulation — and the due date — about four days later than the standard formula would suggest.

Why day 14? Ovulation timing tracks with the length of the luteal phase (the time between ovulation and the next period), which is fairly consistent at about 14 days across most menstrual cycles. Working backward from a predictable next period, day 14 becomes the statistical midpoint for a 28-day cycle.

Method 2: Conception Date

Formula: EDD = Conception date + 266 days

If you know your conception date precisely — often from ovulation-predictor tracking or a single instance of intercourse — this method skips the ovulation assumption entirely and counts forward 266 days, the average length of pregnancy from fertilization to birth.

Method 3: IVF Transfer Date

Formula: EDD = Transfer date + 263 days (Day 3 embryo) or + 261 days (Day 5 embryo)

IVF dating is one of the most precise methods available because the embryo’s exact age is known down to the day. In a Day 3 transfer, the embryo is transferred while it’s still dividing at the cellular level, before reaching the blastocyst stage. In a Day 5 transfer, the embryo has already developed into a blastocyst — a more advanced structure — so fewer days remain until the 266-day conception-based total, which is why the Day 5 formula subtracts two additional days compared to Day 3.

This precision is the reason IVF due dates are considered more reliable than LMP-based estimates: there’s no ovulation-day guesswork involved.

Method 4: First-Trimester Ultrasound

An ultrasound performed in the first trimester — ideally before 13 weeks — is considered the single most accurate dating method, more reliable than LMP even when your cycle history is well known.

How ultrasound dating actually works: In early pregnancy, sonographers measure crown-rump length (CRL), the length of the fetus from the top of the head to the bottom of the torso. CRL grows at a highly predictable rate in early pregnancy, making it an accurate age marker. Later in pregnancy, providers switch to biparietal diameter (BPD) — the width of the head — along with other measurements, since CRL becomes less reliable once the fetus curls and moves.

If a first-trimester ultrasound’s gestational age differs from the LMP-based estimate by more than about 5 to 7 days, providers typically update the due date to match the ultrasound rather than the period-based calculation.

Which Method Should You Use?

Not sure which formula applies to you? Use this quick decision guide.

If you have more than one data point — say, both an LMP and an ultrasound — clinical convention is to defer to the more precise method: ultrasound overrides IVF, IVF overrides conception date, and conception date overrides LMP.

Worked Examples

Example 1: Sarah — Adjusted Cycle Length

Sarah’s last period started on January 1. Her average cycle length is 32 days, not the standard 28.

• Standard LMP + 280 days = October 8
• Cycle-length adjustment: +4 days (32 minus 28)
• Sarah’s estimated due date: October 12

This example shows why the “average cycle length” field matters — skipping it would have given Sarah a due date four days too early.

Example 2: Maria — Known Conception Date

Maria tracked ovulation and knows she conceived on March 15.

• Conception date + 266 days
• Maria’s estimated due date: December 5

Because Maria’s conception date is confirmed rather than estimated, this calculation carries less uncertainty than an LMP-based estimate.

Example 3: Jessica — IVF Day 5 Transfer

Jessica had a Day 5 blastocyst transfer on June 1.

• Transfer date + 261 days
• Jessica’s estimated due date: February 17

Jessica’s due date is considered highly reliable because both the embryo’s age and the transfer date are documented to the day.

Trimester Breakdown and Fetal Development

Pregnancy is divided into three trimesters, each roughly 13 to 14 weeks long. Each trimester brings distinct developmental milestones.

First Trimester (Weeks 1–13): Foundation and Organ Formation

• Weeks 1–4: Fertilization and implantation occur; the earliest pregnancy hormones begin rising.
• Weeks 5–8: The heartbeat becomes detectable by ultrasound around week 6; the neural tube, which becomes the brain and spinal cord, closes.
• Weeks 9–13: All major organs have formed in basic structure. This window includes the nuchal translucency screening, an ultrasound-based test that checks fetal development and screens for certain chromosomal conditions.

Second Trimester (Weeks 14–27): Growth and Movement

• Weeks 14–17: External sex organs become distinguishable on ultrasound; early fetal movement begins, though it usually isn’t felt yet.
• Weeks 18–22: This is the anatomy scan window, a detailed ultrasound checking organ development and growth. Many people first feel movement (“quickening”) during this stretch.
• Weeks 23–27: The fetus reaches the edge of viability — the point at which survival outside the womb becomes possible with intensive medical support — and brain development accelerates rapidly.

Third Trimester (Weeks 28–40+): Final Preparation

• Weeks 28–31: Rapid weight gain begins as fat stores build under the skin.
• Weeks 32–36: Lungs mature toward readiness for breathing air, and the fetus typically settles into a head-down position.
• Weeks 37–40+: This is the full-term window. Pregnancies are classified as early-term (37–38 weeks, 6 days), full-term (39–40 weeks, 6 days), or post-term (past 42 weeks).

Fetal Size by Week

Size comparisons are one of the most commonly searched pregnancy details. Here’s a snapshot across pregnancy.

These figures are population averages. Individual size varies, which is why growth is tracked across multiple visits rather than judged from a single measurement.

Method Comparison Table

Common Mistakes to Avoid

1. Confusing gestational age with fetal age. Gestational age counts from LMP; fetal age counts from conception, roughly two weeks less.
2. Skipping the cycle-length adjustment. As Sarah’s example shows, a non-28-day cycle changes the math meaningfully.
3. Treating the due date as a guaranteed birth date. Only about 5% of babies are born on their exact due date, according to birth-timing data compiled by the CDC’s National Center for Health Statistics; most arrive within a one- to two-week window around it.
4. Ignoring an ultrasound update. If a dating scan shifts your due date, that’s the number your provider will use going forward, not the original LMP calculation.
5. Using an unreliable LMP memory. Irregular periods or recent hormonal birth control use can make LMP-based dating less accurate; flag this for your provider.

Risk Factors and Special Considerations

PCOS (Polycystic Ovary Syndrome). Irregular or absent ovulation associated with PCOS can make LMP-based dating unreliable, since the day-14 ovulation assumption may not hold. An early ultrasound is often recommended for more accurate dating.

Advanced maternal age. Pregnancies in people aged 35 and older are monitored more closely and may include earlier or more frequent dating and growth ultrasounds.

Multiple pregnancy (twins or more). Due-date calculation follows the same formulas, but twin pregnancies are statistically more likely to deliver earlier than the calculated due date, and providers often plan around a full-term window that ends sooner than 40 weeks.

Early pregnancy dating limitations. Ultrasound dating before about 6 weeks is less reliable because the fetus is too small for a precise crown-rump length reading; providers often recommend a follow-up scan a week or two later to confirm dates.

Post-term pregnancy. A pregnancy extending past 42 weeks is considered post-term and typically prompts closer monitoring or a discussion about labor induction, since placental function can decline past this point.

hCG and Early Pregnancy Confirmation

Human chorionic gonadotropin (hCG) is the hormone detected by pregnancy tests and is often the first data point people have before they ever use a due-date calculator. hCG levels roughly double every 48 to 72 hours in early, healthy pregnancy, which is why some providers order repeat blood tests in very early pregnancy to confirm viability before dating begins in earnest. hCG levels themselves aren’t used to calculate a due date, but they frequently prompt the LMP or ultrasound dating this calculator performs.

When to Call Your Provider

Contact your obstetrician, midwife, or care team if you experience heavy bleeding, severe abdominal pain, a significant change in fetal movement after movement has become established, or if your calculated due date differs substantially from what your provider has on record. A due-date discrepancy of more than a week between methods is worth flagging at your next visit, since it can affect the timing of screening tests.

Glossary of Key Terms

• EDD (Estimated Due Date): The calculated date a baby is expected to be born, also sometimes written as EDC (Estimated Date of Confinement) — the two terms mean the same thing.
• LMP (Last Menstrual Period): The first day of your most recent period before pregnancy, used as the standard dating reference point.
• Gestational age: Pregnancy duration counted from the LMP, typically about two weeks ahead of fetal age.
• Fetal (conceptional) age: Pregnancy duration counted from the actual date of conception.
• Trimester: One of three roughly 13-week phases of pregnancy.
• Full-term: A pregnancy delivered between 39 weeks and 40 weeks, 6 days.
• Early-term: Delivery between 37 weeks and 38 weeks, 6 days.
• Post-term: A pregnancy extending past 42 weeks.

Frequently Asked Questions

How accurate is a pregnancy due date calculator?

A due date calculator is accurate as an estimate, not a certainty. Only about 5% of babies are born on their exact due date, and most arrive within a one- to two-week window around it.

Can my due date change during pregnancy?

Yes. If a first-trimester ultrasound shows a gestational age differing from the LMP-based estimate by more than about 5 to 7 days, providers typically update the due date to match the ultrasound.

How many weeks is a full-term pregnancy?

A pregnancy is considered full-term between 39 weeks and 40 weeks, 6 days. Babies born between 37 and 38 weeks, 6 days are considered early-term.

What week am I in if my due date is a certain date?

Count backward 40 weeks from your due date to find your LMP-equivalent starting point, then count forward from there to today’s date. The calculator on this page performs this reverse calculation automatically if you enter your due date instead of your LMP.

Can a pregnancy calculator be wrong by weeks?

Yes, particularly with LMP-based estimates in people with irregular cycles or uncertain period dates. Ultrasound-based dating narrows this margin to just a few days.

Why do IVF due dates differ from LMP due dates?

IVF dating starts from a precisely known embryo age at transfer, removing the ovulation-timing guesswork built into LMP-based Naegele’s Rule calculations. That precision is why IVF and ultrasound dates are generally trusted over LMP dates when they disagree.

What is the Naegele’s Rule formula based on?

Naegele’s Rule assumes a 28-day menstrual cycle with ovulation on day 14 and a 266-day pregnancy from conception, which together produce the 280-day LMP-based formula. It is the dating method endorsed by ACOG for initial estimates.

How is a twin pregnancy due date calculated differently?

Twin pregnancy due dates use the same LMP, conception, or ultrasound formulas described above, but providers plan around delivery statistically occurring earlier than the calculated 40-week date, since twin pregnancies commonly deliver before full term.

What’s the difference between a due date calculator and a pregnancy calculator?

They’re the same tool. “Due date calculator,” “pregnancy calculator,” “due date estimator,” and “gestation calculator” are used interchangeably to describe this type of tool.

What does EDD stand for?

EDD stands for Estimated Due Date. It’s sometimes written as EDC, or Estimated Date of Confinement — an older clinical term meaning the same thing.

What is the difference between an EDD and an EDC?

There is no functional difference — both refer to the calculated due date. EDC is the older clinical phrasing; EDD is the more commonly used modern term.

How early can a due date be calculated?

A due date can be calculated as soon as you know your LMP, which means it’s often available before you even take a pregnancy test. Ultrasound confirmation typically becomes reliable starting around 6 weeks gestational age.

Do doctors use apps or the same manual formulas?

Providers generally use the same formulas described here — Naegele’s Rule, conception-based dating, IVF transfer dating, and ultrasound measurement — whether calculated manually, through an app, or with electronic health record software.

Can stress or lifestyle change my due date?

No. Stress and lifestyle factors don’t change the calculated due date itself, though they can influence cycle regularity, which affects how accurate an LMP-based estimate is in the first place.

Key Takeaways

A pregnancy calculator estimates your due date using one of four methods: last menstrual period, conception date, IVF transfer date, or first-trimester ultrasound. Each method has a different formula and a different level of precision, with ultrasound and IVF dating generally considered the most accurate.

Your due date is a statistical estimate, not a guarantee — only about 5% of babies arrive on that exact day. Use the calculator’s optional fields, especially cycle length and ultrasound data, to sharpen your estimate, and bring any significant date discrepancies to your provider.

For more tools in this cluster, see the ovulation calculator to estimate your fertile window, the conception date calculator to work backward from a known due date, the IVF due date calculator for assisted reproduction timelines, the pregnancy weight gain calculator for trimester-by-trimester guidance, and the fetal development week by week guide for a deeper look at what’s happening at every stage.