Due Date Calculator

Due Date Calculator – LMP, Conception, IVF and Ultrasound-Based Pregnancy Dating

This complete Due Date Calculator helps you estimate your pregnancy due date, current pregnancy week, fetal age, trimester and key milestone dates using four medically recognized methods: the first day of your last menstrual period (LMP), estimated conception date, IVF embryo transfer date, or gestational age from an ultrasound scan. Whether you're newly pregnant, undergoing fertility treatment, or confirming dates with early ultrasound, this tool provides accurate and easy-to-understand results based on standard obstetric rules.

Pregnancy dating can sometimes feel confusing because ovulation, conception and pregnancy week count do not always match the actual developmental age of the fetus. This calculator simplifies everything by aligning your dates with the same methods doctors and midwives use during prenatal visits, giving you a clear timeline for your baby's development and important pregnancy milestones.

How This Due Date Calculator Works

The tool uses clinically accepted timelines and formulas to estimate due dates and pregnancy progression. Depending on which starting information you provide, the calculation adjusts to accurately project your pregnancy timeline. Here is how each method works:

• LMP Method: Uses the date your last menstrual period began, plus 280 days (40 weeks). Adjusts for longer or shorter cycles.
• Conception Date Method: Uses conception or ovulation date and adds 266 days (38 weeks).
• IVF Method: Uses embryo transfer date and embryo age (Day 3 or Day 5) to calculate an exact due date.
• Ultrasound Method: Uses gestational age measured during an ultrasound scan to back-calculate your due date.

No matter which method you choose, the calculator generates a unified pregnancy timeline with trimester placement, fetal age, total gestational age and milestone predictions.

1. Understanding Pregnancy Due Dates

The due date represents the day your pregnancy reaches 40 weeks. Only 4 percent of babies are born on their exact due date, but most births occur within two weeks before or after. The due date is an estimate, not an expiration date, but it provides an essential reference point for prenatal care, testing and planning.

Due dates are based on the gestational age of the pregnancy, counted from the first day of your last menstrual period, not from the day you conceived. This is because conception is often uncertain, while menstrual periods are easier to record and measure.

2. How Pregnancy Is Measured: Gestational Age vs. Fetal Age

Pregnancy is measured in two different ways:

• Gestational age: The age of the pregnancy, measured from the LMP. This is what doctors and pregnancy apps use.
• Fetal age: The actual age of the fetus, typically two weeks younger than gestational age.

For example, when you are considered 6 weeks pregnant, the embryo is typically 4 weeks old. This difference is normal and expected.

3. Method 1 – LMP (Last Menstrual Period) Method

The most widely used method to calculate due date is the LMP method. It assumes:

• You ovulated approximately 14 days after your period began.
• You conceived shortly after ovulation.
• A full-term pregnancy lasts 280 days (40 weeks).

How the LMP Method Works

You enter the first day of your last period. The calculator:

• Adds 280 days (40 weeks).
• Adjusts for cycle length (longer cycles delay ovulation; shorter cycles move it earlier).
• Estimates your conception date.
• Determines your current pregnancy week and trimester.

When the LMP Method Is Most Accurate

• Your cycles are regular.
• You know your last period date with confidence.

If your cycles are irregular or unknown, the ultrasound or conception-based methods may be more precise.

4. Method 2 – Using Conception Date

If you know or suspect your conception date, you can use it to calculate your due date. Conception-based dating assumes that:

• Pregnancy lasts 266 days from conception.
• Ovulation occurred around the time of conception.

This method is particularly useful if:

• You tracked ovulation with OPKs, basal temperature or cervical mucus.
• You had a known timed intercourse or insemination date.
• You used assisted reproduction (medicated cycles).

5. Method 3 – IVF Transfer Date

IVF pregnancies can be dated with extremely high accuracy because embryo development is controlled and recorded. This calculator adjusts due dates differently for:

• Day 3 embryos (cleavage-stage)
• Day 5 embryos (blastocyst-stage)

Medical Rules for IVF Due Date

• Day 5 blastocyst transfer: Due date is transfer date + 261 days.
• Day 3 embryo transfer: Due date is transfer date + 263 days.

These numbers reflect the embryo’s developmental age at the time of transfer.

6. Method 4 – Ultrasound Scan Dating

Early ultrasounds (typically between 6–12 weeks) are extremely accurate for pregnancy dating. If the date of your LMP is uncertain, or if your cycle is irregular, your doctor mayy on ultrasound instead.

How Ultrasound Dating Works

The calculator uses:

• Your ultrasound scan date
• The gestational age measured during the scan

It subtracts the measured gestational age from the scan date to determine an estimated LMP, then calculates your due date from that point.

7. Understanding Pregnancy Weeks

Pregnancy weeks are counted from your LMP:

• Week 1–2: Pre-conception phase (body preparing for ovulation).
• Week 2–3: Ovulation and conception likely occur.
• Week 4: Missed period; early pregnancy symptoms may begin.
• Weeks 5–10: Early embryo development.
• Weeks 11–14: Fetal transition from embryo to fetus.

This is why you may be considered 4 or 5 weeks pregnant shortly after a positive test.

8. Determining Your Trimester

• 1st Trimester: Conception to 13 weeks.
• 2nd Trimester: 14 to 27 weeks.
• 3rd Trimester: 28 weeks to birth.

The calculator automatically determines where you are in the trimester cycle based on your due date.

9. Fetal Age vs Gestational Age

Fetal age is approximately two weeks less than gestational age because pregnancy is counted from your LMP, not conception. For example:

• You are considered 6 weeks pregnant.
• Your fetus is 4 weeks old.

The calculator displays both ages so you understand the difference.

10. Key Pregnancy Milestones

The calculator also estimates major fetal development milestones such as:

• Start of the 2nd trimester (~13 weeks)
• Viability threshold (~24 weeks)
• Start of the 3rd trimester (~28 weeks)

These dates help with planning prenatal testing, travel restrictions and medical appointments.

11. Why Due Dates Are Only Estimates

Although 40 weeks is the standard for estimating due dates, only a small percentage of babies arrive on that exact day. Every pregnancy is unique. Factors that influence delivery timing include:

• Maternal health
• Baby’s development
• Genetics
• Hormonal changes
• Whether this is a first or subsequent pregnancy

Most full-term births occur between 38 and 42 weeks.

12. Howiable Is This Calculator?

This tool uses the same dating rules used in obstetric studies, prenatal clinics and medical guidelines worldwide. Accuracy depends on:

• Correct LMP tracking
• Regularity of menstrual cycles
• Accuracy of ultrasound measurement
• Precision of embryo transfer date (in IVF)

The earlier you calculate your dates in pregnancy, the more accurate the timeline tends to be.

13.ated Pregnancy Tools

• Pregnancy Calculator
• Pregnancy & Conception Calculator
• Ovulation Calculator
• Pregnancy Week Calculator

14. Medical Disclaimer

This tool is for educational and informational purposes only. It should not replace professional medical advice, diagnosis or prenatal care. Always consult your healthcare provider for personalized guidance.