Calculate Your Due Date and Pregnancy Week
Use the pregnancy calculator to estimate the due date, current pregnancy week, gestational age and trimester based on the first day of the last menstrual period (LMP).
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Your Pregnancy Result
This is an estimated pregnancy timeline. Please consult your healthcare professional to confirm your due date and receive personalised guidance.
Pregnancy Timeline
| Stage | Approximate Period |
|---|---|
| First Trimester | Weeks 1–13 |
| Second Trimester | Weeks 14–27 |
| Third Trimester | Weeks 28–40+ |
This calculator is for informational purposes only. It is not a diagnostic tool and should not replace advice from a qualified healthcare professional. Due dates are estimates.
What is a Pregnancy Calculator and How Does it Work?
A pregnancy calculator is a simple tool that estimates the expected due date and current stage of pregnancy. It commonly uses the first day of the last menstrual period (LMP) to estimate how far the pregnancy has progressed and when the baby may be due.
Pregnancy is generally dated from the first day of the LMP and lasts about 40 weeks. This means pregnancy dating begins before conception actually occurs, as the exact date of conception is often difficult to determine.
Based on the information entered, the calculator can provide an Estimated Due Date (EDD), current pregnancy week, gestational age and trimester. These dates provide a useful pregnancy timeline, but they are estimates and may later be confirmed or adjusted by an obstetrician using clinical assessment and ultrasound findings.
How is Your Pregnancy Due Date Calculated?
A pregnancy due date is usually estimated from the first day of the last menstrual period (LMP). By convention, the Estimated Due Date (EDD) is calculated as 280 days, or 40 weeks, from the first day of the LMP. This method assumes a regular 28-day menstrual cycle, with ovulation occurring around day 14.
Calculating the Due Date From Your Last Menstrual Period
A commonly used method, known as Naegele’s rule, estimates the due date by:
First day of the LMP + 7 days − 3 months + 1 year
For example, If the first day of your last period was 1 January 2026, adding 7 days gives 8 January 2026. Subtracting 3 months gives an estimated due date of 8 October 2026.
Because menstrual cycles and ovulation do not always follow a 28-day pattern, the estimate may be less accurate when cycles are irregular or the LMP date is uncertain.
Calculating Pregnancy Weeks and Gestational Age
Gestational age describes how far a pregnancy has progressed and is usually expressed in completed weeks and days. It is counted from the first day of the LMP rather than from the date of conception.
For example, a result of 12 weeks and 3 days means 12 full weeks of pregnancy have been completed and the pregnancy is three days into the 13th week.
Since conception typically occurs around two weeks after the start of the LMP in a regular 28-day cycle, gestational age is usually about two weeks longer than the estimated time since conception.
If periods are irregular, cycle length varies significantly or the LMP date is uncertain, the calculated due date and pregnancy week may be less accurate. In such cases, an obstetrician may use ultrasound findings and other clinical information to confirm gestational age and the Estimated Due Date (EDD).
How Many Weeks Pregnant Am I?
To determine your current pregnancy week, the calculator measures the time elapsed between the first day of your LMP and today's date.
Results are shown in completed weeks and days:
- Example (12 Weeks + 3 Days): This means you have completed 12 full weeks of pregnancy and are currently 3 days into your 13th week.
Understanding your current week helps align your expectations with routine antenatal appointments, blood tests, and ultrasound milestones.
Pregnancy Trimesters and Their Duration
Pregnancy is divided into three trimesters, each representing a different stage of pregnancy and foetal development. Knowing the current trimester can help put the pregnancy week into context and understand how far the pregnancy has progressed. Trimester boundaries can vary slightly between clinical references, but the NHS uses the following week ranges.
First Trimester: Weeks 1–12
The first trimester covers the earliest stage of pregnancy, when the baby's major organs and body systems begin developing. This is also when initial antenatal visits, pregnancy dating and early screening may take place.
Second Trimester: Weeks 13–27
The second trimester begins at week 13 and continues through week 27. During this period, the baby continues to grow and develop, and routine antenatal care generally includes ongoing monitoring and recommended screening or ultrasound examinations.
Third Trimester: Week 28 Until Birth
The third trimester begins at week 28 and continues until birth. During this final stage, the baby gains further weight and matures as the pregnancy progresses towards delivery. Antenatal visits also focus increasingly on maternal and foetal wellbeing and preparation for birth.
The pregnancy calculator uses the estimated gestational age to indicate whether the pregnancy is currently in the first, second or third trimester.
How Accurate is a Pregnancy Due Date Calculator?
A pregnancy due date calculator provides a useful estimate, but it cannot predict the exact date of delivery. Its accuracy depends largely on how certain the last menstrual period (LMP) is and how closely the menstrual cycle follows a regular pattern.
The estimate may be less reliable when ovulation occurs earlier or later than expected, menstrual cycles vary in length, or the exact LMP date is uncertain. For this reason, the calculated due date is usually considered alongside clinical assessment and ultrasound findings.
What If You Have Irregular Periods or Are Unsure of Your LMP?
If periods are irregular or the first day of the last period is difficult to remember, an LMP-based pregnancy calculator may not provide the most reliable estimate of gestational age.
In these situations, an obstetrician may use an ultrasound scan along with the available menstrual and clinical history to establish pregnancy dating more accurately. WHO recommends an ultrasound before 24 weeks of pregnancy, partly to help estimate gestational age.
Can an Ultrasound Change Your Estimated Due Date?
Yes. An ultrasound can sometimes lead to a revision of the estimated due date if the measurements differ sufficiently from the date calculated using the LMP. First-trimester ultrasound is considered the most accurate ultrasound method for establishing or confirming gestational age.
Once an estimated due date has been established using appropriate clinical information and ultrasound findings, any subsequent change should be determined by the obstetrician. Later ultrasound scans are generally used more for monitoring foetal growth and wellbeing than for repeatedly changing the pregnancy due date.
When Should You Consult an Obstetrician During Pregnancy?
Once pregnancy is confirmed, it is advisable to schedule an appointment with an obstetrician to begin antenatal care. Early consultation helps confirm how the pregnancy is progressing, review the estimated due date, discuss appropriate screening and plan follow-up care based on individual health needs.
Consult an obstetrician if there is:
- A positive pregnancy test, to begin antenatal care and plan the recommended check-ups and investigations.
- Uncertainty about the last menstrual period or irregular cycles, particularly if the calculated due date may not be reliable.
- A difference between the calculator-based due date and ultrasound dating, which may need clinical interpretation.
- A previous pregnancy complication or recurrent pregnancy loss, as closer monitoring may be required.
- An existing medical condition, such as diabetes, high blood pressure or a thyroid disorder, that may affect pregnancy care.
- Any concerning symptoms during pregnancy, including bleeding, severe or persistent pain, severe headache, visual changes, breathing difficulty or other symptoms that cause concern.
- Questions about antenatal screening, nutrition, medicines or pregnancy progression.
At Graphic Era Hospital, our Obstetrics & Gynaecology team supports women throughout pregnancy with antenatal check-ups, ultrasound and screening, high-risk pregnancy care, nutritional guidance and delivery planning. The team can also help clarify pregnancy dating when the LMP-based estimate is uncertain and guide the next steps based on the mother’s health and how the pregnancy is progressing.
To book a consultation with an obstetrician at Graphic Era Hospital, simply call 1800 889 7351.
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Frequently Asked Questions (FAQs)
How does a pregnancy calculator estimate my due date?
A pregnancy calculator usually estimates the due date from the first day of the last menstrual period (LMP). Pregnancy is conventionally dated as approximately 40 weeks, or 280 days, from this date. The result is an estimate and may be confirmed or revised using clinical assessment and ultrasound findings.
How many weeks are there in a pregnancy?
Pregnancy is generally dated as approximately 40 weeks from the first day of the LMP. The actual length of pregnancy can vary, so the estimated due date should be viewed as a reference point rather than an exact delivery date.
What does gestational age mean?
Gestational age describes how far a pregnancy has progressed and is expressed in weeks and days. It is usually counted from the first day of the LMP rather than from the estimated date of conception.
Can I calculate my due date if I have irregular periods?
A calculator can still provide an estimate, but LMP-based dating may be less reliable when menstrual cycles are irregular or the LMP is uncertain. In such situations, an obstetrician may use ultrasound findings and clinical information to establish gestational age more accurately.
Can an ultrasound change my estimated due date?
Yes. An ultrasound may lead to a revised estimated due date when the measurements differ sufficiently from LMP-based dating. First-trimester ultrasound is the most accurate ultrasound method for establishing or confirming gestational age.
Can a pregnancy calculator tell the exact date my baby will be born?
No. A pregnancy calculator provides an Estimated Due Date (EDD) rather than an exact date of delivery. The timing of labour varies between pregnancies, so the calculated date should be used as a general guide.
What if I do not remember the first day of my last period?
If the LMP date is uncertain, avoid relying solely on an online calculator for pregnancy dating. An obstetrician can review the available history and use ultrasound measurements to help establish gestational age and the estimated due date. WHO recommends an ultrasound before 24 weeks of pregnancy, partly to estimate gestational age.
Is pregnancy dating different after IVF?
Yes. When pregnancy results from IVF or another assisted reproductive technology (ART), the estimated due date is based on ART-derived gestational age rather than relying on the LMP alone. For IVF pregnancies, the age of the embryo and the date of embryo transfer are used when establishing the due date.
What should I do if my calculated due date differs from my ultrasound due date?
Rely on the date determined by your obstetrician. Doctors use clinical guidelines (such as ACOG recommendations) to select the most reliable date.
When should I consult an obstetrician after finding out I am pregnant?
You should consult an obstetrician as soon as you find out that you are pregnant. At Graphic Era Hospital, our Obstetrics & Gynaecology team provides antenatal care and pregnancy support, with assessment and monitoring planned according to your individual health and pregnancy needs.
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