Due Date Calculator by Last Menstrual Period

LMP due date calculator

An LMP due date calculator uses the first day of your last menstrual period to estimate when your baby is due. It applies Naegele's rule — adding 280 days (40 weeks) to your LMP — and adjusts for your actual cycle length if it differs from 28 days. This is the same method doctors and midwives use at your first prenatal appointment.

What does LMP mean?

LMP stands for Last Menstrual Period. It refers to the very first day of bleeding from your most recent period — not the last day, and not light spotting that sometimes happens before full flow. This date is used as the starting point for pregnancy dating because most women remember it more easily than the exact day of conception, which is harder to pinpoint.

When you visit a healthcare provider, one of the first questions they ask is: "What was the first day of your last period?" That is your LMP. From there, they calculate your gestational age and estimated due date.

How to calculate due date from last menstrual period

The standard method for calculating your due date from your LMP is called Naegele's rule. Here is how it works step by step:

1. Identify the first day of your last menstrual period.
2. Add 280 days (40 weeks) to that date.
3. If your average cycle is longer than 28 days, add the difference. If shorter, subtract the difference.

For example, if your LMP was January 1 and your cycle is 28 days, your due date is October 8. If your cycle is 32 days, your due date is October 12. The calculator above does this automatically.

Formula: Due date = LMP + 280 days + (cycle length − 28)

Why doctors use LMP to calculate due date

Doctors use LMP because it is a reliable, consistent, and easy-to-obtain starting point. Most women can recall the first day of their period, while very few know the exact moment of conception. LMP-based dating also aligns with how pregnancy weeks are counted clinically: week 1 begins on the first day of your last period, even though conception has not yet occurred.

While an early ultrasound is the most accurate way to date a pregnancy, LMP remains the first-line method because it is available immediately and works well for women with regular cycles.

LMP vs conception date

LMP and conception date are not the same. In a typical 28-day cycle, conception happens approximately 14 days after the first day of your LMP. However, ovulation — and therefore conception — can occur earlier or later depending on your cycle length.

If you know your conception date, your due date is 266 days (38 weeks) after that day. This is more precise than LMP but only useful if you were tracking ovulation or fertility treatments. For most women, LMP is the practical starting point.

What if I do not know my LMP?

If you cannot remember the first day of your last period, there are still ways to estimate your due date:

• First-trimester ultrasound — the most accurate alternative. Measures the embryo and estimates gestational age within 3–5 days.
• Ovulation tracking — if you used LH strips, BBT, or cervical mucus monitoring, your ovulation date plus 266 days gives a good estimate.
• Conception date — if you know when you had intercourse or a fertility treatment, add 266 days.
• Pregnancy test timing — a very rough guide. Most home tests turn positive around 4 weeks from LMP.