Baby Due Date Calculator – Estimate Your Pregnancy Timeline

What Does This Calculator Do?

Finding out you’re pregnant is one of life’s biggest moments — and one of the first questions that follows is almost always: when is the baby due? This Baby Due Date Calculator gives you that answer right away, along with a full picture of your pregnancy timeline broken down by trimester milestones. It works in two ways depending on what information you have available, so you’re not stuck if you don’t know your exact last menstrual period date or your conception date.

The LMP method — using the first day of your last menstrual period — is the most commonly used approach and the one most healthcare providers rely on in early pregnancy. It’s the standard starting point for gestational age calculations because it’s a date most women can recall accurately, even before they know they’re pregnant. The Conception Date method works from the date fertilization is believed to have occurred, which gives a slightly different but equally valid calculation for women who know or have tracked their ovulation closely.

Whichever method you choose, the calculator gives you your estimated due date (EDD), your current gestational age in weeks, and the dates for your key trimester milestones — when the first trimester ends at 13 weeks and when the second trimester ends at 27 weeks. All of this is laid out in a clean milestone table and a visual pregnancy timeline chart that shows the full 40-week journey with trimester stages color-coded and your current progress highlighted, so you can see exactly where you are in the pregnancy at a glance.

The Formula Behind It

The two calculation methods use slightly different gestation periods, both of which are standard in obstetric practice:

LMP Method — Naegele’s Rule:

The most widely used formula for calculating the estimated due date from the last menstrual period is Naegele’s Rule:

EDD = LMP + 280 days (40 weeks)

The 280-day figure accounts for the full gestational period counting from the first day of the last menstrual period — which includes approximately 2 weeks before conception actually occurs, since ovulation typically happens around day 14 of a 28-day cycle.

For cycles that differ from the standard 28 days, the calculator adjusts the ovulation estimate accordingly:

Adjusted EDD = LMP + 280 days + (Cycle Length − 28 days)

So for a woman with a 30-day cycle, the EDD is pushed forward by 2 days compared to the standard calculation. For a 26-day cycle, it’s moved back by 2 days.

For example, LMP of March 1, 2025 with a standard 28-day cycle:

• EDD = March 1 + 280 days = December 5, 2025
• First Trimester Ends (13 weeks): March 1 + 91 days = June 1, 2025
• Second Trimester Ends (27 weeks): March 1 + 189 days = September 7, 2025

Conception Date Method:

When the conception date is known, the formula uses a shorter gestation period because conception itself occurs approximately 2 weeks into the standard LMP-based calculation:

EDD = Conception Date + 266 days (38 weeks)

For example, a conception date of March 15, 2025:

• EDD = March 15 + 266 days = December 6, 2025
• First Trimester Ends: March 15 + 77 days = May 31, 2025
• Second Trimester Ends: March 15 + 175 days = September 6, 2025

Current Gestational Age is calculated as:

Gestational Age = Today’s Date − LMP (or Conception Date adjusted)

This is expressed in completed weeks and days, updated automatically based on today’s date so it always reflects your current stage of pregnancy.

How to Use It

Getting your due date and pregnancy timeline takes just seconds:

1. Select your Calculation Method — Last Menstrual Period (LMP) or Conception Date
2. Enter the relevant Date — first day of your last period, or your estimated conception date
3. If using LMP, enter your Cycle Length in days (21–35, default is 28)
4. Click Calculate to see your results
5. Review your estimated due date, current gestational age, trimester milestone dates, the full event table, and the pregnancy timeline chart

Why It’s Worth Using

Knowing your due date is more than just a countdown — it’s the anchor point for every prenatal appointment, trimester milestone, and planning decision throughout your pregnancy. It’s the date your healthcare provider will use to track fetal development, schedule screenings, and assess whether your pregnancy is progressing normally. Having a reliable estimate from day one helps you stay informed and prepared throughout the entire journey.

The trimester milestone dates are particularly useful for planning. Knowing exactly when your first trimester ends helps you understand when the risk of miscarriage drops significantly and when many women feel comfortable sharing their news. Knowing when the second trimester ends marks the beginning of the third trimester — the home stretch — and helps you plan maternity leave, hospital tours, and baby preparations around a realistic timeline.

The cycle length adjustment in the LMP method is a small but meaningful detail that most basic due date calculators skip entirely. For women with longer or shorter cycles, the standard 28-day assumption can put the due date off by several days — and this calculator corrects for that automatically.