A pregnancy calculator helps estimate where you are in pregnancy and when your baby may be due. Depending on the information you already have, pregnancy dating can be based on the first day of your last menstrual period (LMP), a known conception date, an ultrasound-reported gestational age, an IVF embryo transfer date, or another established pregnancy-date anchor.
This calculator brings several dating methods into one tool. It can estimate your expected due date (EDD), conception timing, gestational age, days pregnant, days remaining, trimester, a generalized birth window, week-by-week development information, and pregnancy milestones.
Because pregnancy dating is a clinical process rather than a simple calendar prediction, different inputs can produce different estimates. Your official estimated due date should be established by your healthcare professional using the best available clinical information.
How Does a Pregnancy Calculator Work?
A pregnancy calculator estimates important pregnancy dates from a known dating point. The most common starting point is the first day of the last menstrual period, or LMP.
For conventional LMP dating, the estimated due date is approximately 280 days, or 40 weeks, from the first day of the LMP. This convention assumes a typical 28-day cycle and ovulation around cycle day 14. Real menstrual cycles and ovulation timing vary, so an LMP-based date is an estimate rather than a guarantee of when delivery will occur.
This calculator goes beyond a simple LMP formula by supporting multiple dating methods. You can work forward from an LMP or conception date, work backward from an expected due date, use an ultrasound-reported gestational age, or use IVF embryo-transfer information.
Pregnancy is conventionally dated from the first day of the last menstrual period, so gestational age includes approximately two weeks before conception in a typical cycle.
What Information Do I Need to Calculate My Pregnancy Dates?
The information required depends on the dating method you choose.
| Dating Method | Information Needed | Main Use |
|---|---|---|
| LMP | First day of last period | Conventional pregnancy dating |
| Due Date | Estimated delivery date | Work backward through timeline |
| Conception | Known/estimated conception | Conception-based dating |
| Ultrasound | Scan date + reported GA | Ultrasound-based timeline |
| IVF Transfer | Transfer date + embryo stage | ART dating |
| Custom Start | Gestational anchor date | Custom timeline |
| Reverse Due Date | Target due date | Reverse dating |
How Is a Due Date Calculated From the Last Period?
For conventional LMP dating, the estimated due date is approximately 280 days from the first day of the last menstrual period.
This is the traditional 40-week pregnancy-dating framework.
The important limitation is that the LMP does not tell you the exact day ovulation or fertilization occurred. The traditional calculation assumes a typical cycle, so differences in cycle length or ovulation timing can change the actual timing of conception.
The calculator can incorporate cycle information where supported, but the result should still be treated as an estimate.
What Is Gestational Age?
Gestational age describes how far a pregnancy has progressed. It is commonly expressed in weeks and days.
For example, 35w0d means 35 completed weeks and 0 additional days.
Gestational age is conventionally counted from the first day of the LMP rather than from fertilization. That means the first approximately two weeks of the standard pregnancy timeline occur before conception in a typical cycle.
This is why a person may be described as four weeks pregnant even though fertilization occurred roughly two weeks earlier.
Gestational age: time counted from LMP
Age since conception: time since fertilization
These are different measurements.
What Is Naegele's Rule?
Naegele's rule is the traditional calendar method for estimating an expected due date from the first day of the LMP.
The classic expression is: LMP + 1 year − 3 months + 7 days, which corresponds approximately to LMP + 280 days.
LMP: January 1
Estimated due date: October 8
The method is useful as a conventional starting point, but it does not capture every variation in menstrual cycles or ovulation.
Why Is My Due Date Only an Estimate?
An estimated due date is a reference point rather than a promise that birth will occur on that exact day.
The date can differ from the actual delivery date because pregnancy length varies between individuals and because the initial dating method may not perfectly capture ovulation, fertilization or early fetal development.
ACOG uses the estimated due date as the reference point for establishing gestational age and organizing pregnancy care. An established EDD should not be repeatedly changed simply because a later estimate differs slightly.
LMP Dating vs Ultrasound Dating
LMP dating can work well when the date is known accurately and menstrual cycles are reasonably predictable.
It becomes less reliable when:
- the LMP is uncertain
- cycles are irregular
- ovulation occurs earlier or later than expected
- recent hormonal contraception affects cycle interpretation
ACOG states that first-trimester ultrasound measurement of the embryo or fetus is the most accurate method for establishing or confirming gestational age.
That does not mean every later ultrasound should produce a new due date. The clinical significance of a dating difference depends on when the ultrasound was performed and how large the discrepancy is.
If you already have a gestational age reported by an ultrasound, use this calculator's Ultrasound Scan mode rather than replacing that information with a generic calendar estimate.
How Does Ultrasound Dating Work?
The Ultrasound Scan mode uses two pieces of information: the date of the ultrasound and the gestational age reported by the scan.
The calculator then works backward to estimate the corresponding pregnancy dating and forward to estimate the EDD.
This is date arithmetic based on the gestational age you enter. It does not independently measure an ultrasound image and should not be interpreted as a substitute for a clinical ultrasound examination.
For pregnancy dating, early ultrasound is generally more precise than later ultrasound dating.
How Does IVF Pregnancy Dating Work?
IVF pregnancies can be dated more precisely because the embryo's developmental stage and transfer date are known.
For example, ACOG's pregnancy-dating guidance uses:
- Day-3 embryo transfer: approximately 263 days from transfer to the EDD
- Day-5 embryo transfer: approximately 261 days from transfer to the EDD
The calculator's IVF Transfer mode combines the transfer date with the embryo stage to establish the pregnancy timeline. This approach is different from simply entering an estimated LMP.
Why Are the First Two Pregnancy Weeks Different?
Pregnancy dating starts before fertilization. Week 1 begins on the first day of the LMP. Week 2 generally corresponds to the period leading toward ovulation.
If fertilization occurs, conception follows ovulation. This means the conventional gestational-age clock can reach approximately two weeks around the time conception occurs.
Those early weeks should not be interpreted as though an embryo or fetus already exists from the first day of the LMP.
The calculator therefore distinguishes the pregnancy timeline from the biological age of the embryo or fetus.
What Do the Pregnancy Trimesters Mean?
Pregnancy is commonly divided into three trimesters:
These divisions help organize pregnancy care and developmental information. The exact milestone timing of an individual pregnancy can vary, so trimester labels should be understood as gestational-age categories rather than exact developmental boundaries.
What Happens During Each Stage of Pregnancy?
Early pregnancy includes implantation and rapid embryonic development.
During the first trimester, major structures begin to develop and early pregnancy changes become apparent. The second trimester is characterized by continued structural development, growth and increasing fetal movement.
During the third trimester, fetal growth continues while organs such as the lungs and brain continue to mature.
The calculator's weekly development information is designed to provide general context for these stages. It should not be used as a diagnostic measure of an individual fetus.
How Should I Read the Week-by-Week Development Information?
The calculator's week-by-week development section connects your estimated gestational age with general developmental information.
It may include developmental milestones, size references, weight references, maternal changes, and pregnancy-stage information.
These are educational reference points. They are not ultrasound measurements of your individual pregnancy. Fetal growth varies naturally between pregnancies, so a reference value should not be interpreted as a required measurement for your baby.
What Does the Fetal Growth Chart Show?
The fetal-growth chart provides a visual reference for how fetal size changes across pregnancy.
Its purpose is educational: it helps you understand the general direction of growth from early pregnancy toward term.
The chart should not be interpreted as a prediction of the exact weight of your baby at a particular week. Individual fetal growth is assessed clinically using gestational age, ultrasound measurements and growth patterns over time.
What Is the Difference Between an Estimated Due Date and a Birth Window?
The estimated due date is a single reference date. A birth window is broader and is intended to illustrate that delivery does not normally occur on one universally fixed day.
These labels describe gestational age and should not be interpreted as a guarantee that a particular pregnancy will continue to any specific week.
How Does Pregnancy Dating Differ for Twins and Triplets?
Twin and triplet pregnancies require different clinical management from singleton pregnancies.
The calculator supports Single Baby, Twins, and Triplets. Pregnancy-type selection affects the calculator's applicable pregnancy timeline and generalized delivery planning.
However, multiple pregnancies are more complex than a singleton pregnancy. Factors such as fetal growth, placental arrangement, maternal complications and other pregnancy-specific findings can affect management.
The calculator's generalized output should therefore be treated as educational planning information rather than individualized delivery advice.
Why Does Accurate Pregnancy Dating Matter?
Many prenatal tests and appointments are scheduled according to gestational age.
- first-trimester screening
- dating ultrasound
- anatomy ultrasound
- glucose screening
- later-pregnancy testing
- delivery planning
For example, ACOG recommends that a second-trimester ultrasound for assessment of fetal structural defects is ideally performed between 18 and 22 weeks. Accurate dating therefore helps place these events in the correct part of the pregnancy timeline.
What If This Calculator Gives Me a Different Due Date Than My Doctor?
Do not automatically assume the calculator is more accurate.
A clinical due date may be based on reliable LMP dating, first-trimester ultrasound, assisted reproductive technology, or clinical history.
ACOG recommends using the best obstetric estimate and avoiding unnecessary changes to an established EDD. If your calculator result differs from the date documented by your healthcare provider, use the clinically established date for medical care.
What Can This Pregnancy Calculator Tell Me?
The calculator can estimate:
It also lets you explore the pregnancy timeline using multiple dating methods rather than forcing every pregnancy into an LMP-only calculation.
What Can't a Pregnancy Calculator Tell Me?
A pregnancy calculator cannot diagnose pregnancy complications or determine whether a pregnancy is healthy.
It cannot reliably determine:
- exact conception time
- exact delivery date
- fetal abnormalities
- fetal growth disorders
- pregnancy complications
- whether a symptom is dangerous
- whether a medication is safe
- whether a prenatal test will be positive or negative
Those questions require clinical assessment.
Medication Safety in Pregnancy: The FDA PLLR Framework
Older prescription-drug labeling sometimes used pregnancy letter categories such as A, B, C, D and X. The FDA removed those categories from prescription drug labeling under the Pregnancy and Lactation Labeling Rule because the letter system could oversimplify pregnancy risks. Current labeling instead uses narrative risk and clinical information.
Do not use this calculator as a medication-safety checker.
When Should I Contact My Healthcare Provider?
Do not use a calculator to decide whether urgent symptoms can be ignored. Contact your maternity or prenatal care team for concerns such as:
- vaginal bleeding
- leaking fluid
- severe abdominal or pelvic pain
- severe headache or vision changes
- significant shortness of breath
- concerning swelling
- persistent vomiting or inability to keep fluids down
- reduced fetal movement later in pregnancy
- suspected preterm labor
- any symptom your healthcare team has told you requires urgent assessment
How to Use This Pregnancy Calculator
Start by choosing the dating method for which you have the most reliable information.
- Select LMP, Due Date, Conception Date, Ultrasound Scan, IVF Transfer, Custom Start or Reverse Due Date.
- Enter the required date or gestational information.
- Select Single Baby, Twins or Triplets when appropriate.
- Review the estimated due date.
- Review gestational age, days pregnant, days remaining and trimester.
- Explore the weekly development information.
- Review the pregnancy timeline and milestone information.
- Use the PDF, print, copy, share or CSV tools when you need to save the result.
Clinical References
The pregnancy-dating information on this page is based primarily on guidance from the American College of Obstetricians and Gynecologists and other authoritative health agencies.
Pregnancy Calculator FAQ
Frequently asked clinical and mathematical questions about pregnancy calculation, gestational age, and due date estimation.
Medical Disclaimer
This pregnancy calculator provides estimated pregnancy dates, gestational-age information and educational pregnancy references. It is not a diagnostic tool and does not replace prenatal care.
The estimated due date and other dates depend on the information entered and the dating method used. Clinical pregnancy dating may take into account menstrual history, ultrasound findings, assisted reproductive technology and other medical information.
Do not use this calculator to diagnose pregnancy complications, interpret an ultrasound independently, determine medication safety, change treatment, or delay urgent medical care.
When a pregnancy date from this calculator differs from the date established by your healthcare professional, use the clinically established date for medical care.