Is an ultrasound scan more accurate than LMP for dating pregnancy?

Yes. Early ultrasound scans — especially those done before 13 weeks 6 days — are generally more accurate for dating pregnancy than the LMP method. This is because not all women have regular 28-day cycles. If your scan EDD differs from your LMP-based EDD, your doctor will usually use the scan date for pregnancy management.

Can I use this calculator if I do not know my LMP date?

If you do not know your LMP date, you can use your scan EDD instead. Select 'Yes' when asked about a scan report and enter the EDD shown on your scan. If you have neither, an early ultrasound scan is the most reliable way to date the pregnancy.

What does the due date mean?

The due date (EDD — Estimated Delivery Date or Expected Due Date) is the estimated date when your baby may be born. It is calculated as 40 weeks (280 days) from the first day of the last menstrual period. Only about 5% of babies are born on their exact due date — most births occur between 37 and 42 weeks.

Pregnancy Due Date Calculator | Pregnancy Week Calculator

Q: What should I do if my pregnancy goes past the due date?

Going past the due date is common. Most women give birth within 1–2 weeks of their due date. If you reach 41 or 42 weeks, your doctor will assess whether induction of labour is recommended. Do not wait without medical follow-up after 40 weeks — keep your antenatal appointments.

🤰 What does this tool do?

This calculator estimates your expected delivery date (EDD) and shows your current gestational age — how many weeks and days pregnant you are today.

You can use either:

The first day of bleeding of your last menstrual period (LMP method)
The EDD shown on your ultrasound scan report (scan EDD method)

The result also shows key pregnancy milestones with estimated dates.

Do you have an ultrasound scan report that gives an expected delivery date?

No — I do not have a scan EDD Yes — I have a scan EDD

Estimated Due Date

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Pregnancy

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weeks

Trimester

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Days Until EDD

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days

Key Pregnancy Milestones

⚠️ This calculator is not for emergencies

Seek medical advice urgently if you have heavy bleeding, severe abdominal pain, fainting, severe vomiting, fever, fluid leakage, or reduced fetal movements later in pregnancy.

⚠️ Medical Disclaimer

This is an estimate only. Your doctor may adjust your due date based on ultrasound findings, clinical assessment, and the regularity of your menstrual cycle. Attend all recommended antenatal visits.

Understanding Your Due Date and Pregnancy Weeks

Your due date is an estimate of when your baby may be born. Most babies arrive within two weeks before or after this date.

How is the due date calculated?

The standard method (Naegele's rule) adds 280 days (40 weeks) to the first day of the last menstrual period. This assumes a regular 28-day cycle. If your cycle is longer or shorter, or if your periods are irregular, your doctor may prefer to use an ultrasound scan date instead.

Why is the first day of bleeding so important?

The first day of bleeding is the reference point for the entire pregnancy calendar. Even though conception typically occurs around day 14 of the cycle, pregnancy is measured from the first day of the last period — not from the date of conception. Entering the wrong date — such as the last day of bleeding instead of the first — will shift every calculation by several days and affect your milestone dates.

LMP vs scan EDD — which should I use?

Early ultrasound scans — especially those done before 13 weeks 6 days — are generally more accurate for dating pregnancy, because they measure the actual size of the embryo rather than relying on cycle assumptions. International guidelines (ACOG, RCOG) recommend that if a first-trimester scan EDD differs from the LMP-based EDD by more than 7 days, the scan date should be used. If you have an early scan EDD, use it in this calculator for a more accurate result.

What do the pregnancy trimesters mean?

Pregnancy is divided into three trimesters. The first trimester (weeks 1 to 13 weeks 6 days) covers early development — major organ formation occurs during this period and miscarriage risk is highest. The second trimester (weeks 14 to 27 weeks 6 days) is typically the most comfortable phase — fetal movements begin and the anomaly scan is usually done. The third trimester (28 weeks onwards) is the final growth phase leading up to delivery.

Limitations of this tool

This is an educational estimation tool. It cannot account for cycle irregularity, multiple pregnancies, IVF conception dates, or clinical examination findings. Your doctor may change your due date based on scan measurements. Always follow your doctor's recommended due date, not this calculator's estimate, for all clinical decisions.

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Frequently Asked Questions

Common questions about pregnancy dating and due dates, answered by Sineth Hospitals.

What is the LMP date and how do I find it? ▼

LMP stands for Last Menstrual Period. The LMP date is the first day of bleeding of your most recent menstrual period — not the last day. You can find this in your diary, phone calendar, or period tracking app. If you are not sure, an early ultrasound scan is the most reliable way to date the pregnancy.

My scan EDD is different from my LMP EDD — which is correct? ▼

If an early scan (done before 13 weeks 6 days) shows a different EDD from the LMP-based calculation, international guidelines recommend using the scan EDD. Early scans measure the actual size of the embryo, which is more accurate than assuming a standard 28-day cycle. Tell your doctor about both dates and they will decide which to use for your pregnancy management.

What does it mean if I am past my due date? ▼

Going past the due date is common. Only about 5% of babies are born on their exact due date. Most births occur within 1–2 weeks of the EDD. If you reach 41 or 42 weeks, your doctor will assess whether induction of labour is recommended. Do not wait without medical follow-up after 40 weeks — keep your antenatal appointments and contact your doctor if you notice reduced fetal movements.

How accurate is this due date calculator? ▼

This calculator uses standard formulas (LMP + 280 days, or from scan EDD) which are the same methods used by doctors. However, the accuracy depends on the accuracy of the date you enter. If your LMP date is uncertain, if your cycle is irregular, or if you have a multiple pregnancy, the result may be less accurate. An early ultrasound scan is the gold standard for pregnancy dating.

When should I have my first scan? ▼

An early dating scan is typically done between 11 and 13 weeks 6 days. This is also when the nuchal translucency (NT) measurement is taken to screen for chromosomal conditions. Some centres offer an early viability scan at 6–8 weeks to confirm the pregnancy location and heartbeat. Your doctor will advise on the timing based on your situation.