lmp and edd calculator

What is an LMP and EDD calculator?

An LMP and EDD calculator estimates pregnancy timing using the first day of your last menstrual period (LMP). From that date, it projects your estimated due date (EDD), current gestational age, and time remaining in pregnancy.

This is one of the most common starting methods in prenatal care because it is simple and fast. Most calculators use a standard pregnancy length of 280 days (40 weeks) from LMP, then adjust for cycle length when needed.

How the due date is calculated

Naegele's rule (the classic approach)

In simple terms, due date estimation is often:

• Add 1 year to LMP
• Subtract 3 months
• Add 7 days

Digitally, this is equivalent to adding 280 days to LMP. If your menstrual cycle is longer or shorter than 28 days, many calculators adjust the estimate accordingly.

Cycle length adjustment

If your cycle is 32 days, ovulation may occur later than in a 28-day cycle, so the due date may be shifted forward. If your cycle is shorter (for example 25 days), due date may be shifted slightly earlier.

What your result means

After calculation, you typically see:

• Estimated Due Date (EDD): The projected date at 40 weeks gestation.
• Estimated conception window: A rough estimate based on cycle timing.
• Current gestational age: Weeks and days since LMP.
• Trimester: First, second, or third based on current week.

Remember: only a small percentage of babies are born exactly on their due date. Birth often happens in a healthy range around that date.

Why your date may change later

An LMP-based estimate is useful, but your care team may revise dating using early ultrasound, especially if:

• You are unsure of your LMP date.
• Your periods are irregular.
• You recently stopped hormonal contraception.
• Conception timing is uncertain.

First-trimester ultrasound often gives the most accurate pregnancy dating when LMP is unclear.

Trimester timeline at a glance

First trimester (0 to 13 weeks)

• Initial prenatal labs and health review
• Possible early dating ultrasound
• Nutrition, folic acid, and lifestyle planning

Second trimester (14 to 27 weeks)

• Anatomy scan (usually around 18 to 22 weeks)
• Monitoring growth, blood pressure, and symptoms
• Discussion of movement, sleep, and activity

Third trimester (28 to 40+ weeks)

• More frequent visits
• Birth preparation and postpartum planning
• Monitoring baby position and late-pregnancy wellbeing

Tips for using this calculator accurately

• Use the first day of bleeding for your LMP, not the last day.
• Enter your average cycle length from recent months.
• If your cycle is highly irregular, consider this an approximate estimate only.
• Confirm dating with your prenatal provider.

Frequently asked questions

Is the EDD an exact delivery date?

No. It is a clinical estimate. Many healthy births occur before or after that day.

Can I use this if I conceived through IVF?

IVF pregnancies are usually dated using embryo transfer date rather than LMP alone. Follow your fertility clinic's dating method.

What if I do not remember my LMP?

Use your best estimate for a rough date, but rely on an early ultrasound for accurate dating.

When to contact a clinician

Always seek medical care for concerning symptoms such as severe pain, heavy bleeding, fainting, persistent vomiting, fever, or reduced fetal movement later in pregnancy. This calculator is educational and does not replace professional diagnosis or treatment.