Pregnancy Due Date Calculator

How this pregnancy due date calculator works

This calculator estimates your baby’s due date (also called the estimated date of delivery, or EDD) and key pregnancy milestones using the first day of your last menstrual period (LMP). It follows the standard method used in many clinics, often called Naegele’s rule. The result is an informed estimate — not a guarantee of when labor will start.

Healthcare professionals count pregnancy in gestational weeks, starting from the first day of your LMP, not from the day you actually conceived. That means when you are told you are “6 weeks pregnant,” it has typically been about four weeks since conception, assuming a regular 28-day cycle.

By entering a single date (your LMP), the calculator can:

• Estimate your baby’s due date.
• Approximate a likely conception window.
• Outline trimester boundaries and milestone weeks.
• Provide a simple schedule you can discuss with your prenatal care team.

Naegele’s rule: the standard due date formula

Naegele’s rule is a long-standing way to estimate a due date based on the LMP in a person with a typical 28-day menstrual cycle. It assumes that ovulation and conception happen about 14 days after the period begins and that pregnancy lasts about 280 days (40 weeks) from the LMP.

In simple terms:

Estimated due date = first day of your last period + 280 days

In MathML form, the relationship between LMP and the estimated due date can be written as:

Where:

• LMP is the first day of your last menstrual period.
• EDD is the estimated date of delivery, also called the due date.

The calculator uses this 280-day convention to build your pregnancy timeline, including trimester start and end points. Many professional organizations, such as national obstetrics and gynecology societies, reference this same standard when discussing term pregnancy.

Adjusting for different cycle lengths

Naegele’s rule assumes a 28-day cycle. If your cycle is consistently shorter or longer, ovulation may occur earlier or later, which can shift the estimated due date. A simple way to approximate this is to adjust based on how many days your cycle differs from 28.

Let:

• c = your average cycle length in days
• LMP = first day of your last menstrual period
• D_a = adjusted estimated due date

Then you can estimate:

In words: start with 280 days, then add the number of days your cycle is longer than 28, or subtract the number of days it is shorter.

Examples of how cycle length can influence the estimate include:

Many clinicians will combine this kind of cycle-based adjustment with early ultrasound measurements to refine your official due date, especially if your cycles are not close to 28 days.

Worked example: from LMP to due date and trimesters

To see how the calculator uses these ideas in practice, imagine your last menstrual period began on March 1.

1. Estimate the due date.
   • Start date (LMP): March 1
   • Add 280 days (40 weeks)
   • The estimated due date falls around December 6.
2. Approximate conception window.
   • For a 28-day cycle, ovulation is around day 14 after LMP (around March 15).
   • Conception most often happens in the few days before and just after ovulation.
   • So a likely conception window might be roughly March 12–17.
3. Trimester boundaries (gestational age).
   • First trimester: LMP to about 13 weeks 6 days (early June).
   • Second trimester: about 14 to 27 weeks (early June through early September).
   • Third trimester: about 28 weeks to birth (early September onward).

The calculator automates this math for whatever LMP date you provide and presents the results as a simple schedule you can print, copy, or bring to appointments.

Understanding what an estimated due date really means

An estimated due date is best thought of as the middle of a normal delivery window, not as a fixed deadline. Only a small percentage of babies are born exactly on their due date. Many studies suggest that, for spontaneous (uninduced) labor in otherwise uncomplicated pregnancies:

• Most births occur between about 37 and 41 weeks of gestation.
• Going slightly before or after the estimated date can still be completely normal.

Healthcare teams often describe pregnancies this way:

Your personal situation may be different. Underlying health conditions, multiple pregnancy (twins or more), and your baby’s growth pattern can all influence how your provider interprets the due date and when they might recommend delivery.

How providers refine due dates over time

Your initial due date usually comes from your LMP, but as pregnancy progresses, your prenatal care team may adjust it. Common reasons include:

• Uncertain or irregular cycles. If you are not sure of your LMP, or your cycles vary a lot from month to month, the LMP-based estimate may be less accurate.
• Early ultrasound measurements. An ultrasound in the first trimester, especially between about 7 and 13 weeks, can measure the embryo or fetus (often using crown-rump length) and estimate gestational age. If this differs significantly from the LMP-based estimate, your official due date may be changed.
• Assisted reproduction. With IVF, ICSI, or some IUI cycles, clinics know exactly when fertilization or embryo transfer occurred. In these cases, due dates are usually calculated from that known date rather than from the LMP.

If your provider tells you a different due date than this calculator shows, their date should be considered more reliable because it incorporates more individualized information and clinical judgment.

Using your due date for planning

Once you have an estimated due date and milestone schedule, you can use it to organize many aspects of pregnancy and early parenting. Examples include:

• Prenatal appointments and tests. Many screening tests and checkups are tied to specific gestational weeks (for example, nuchal translucency scans in the late first trimester, anatomy scans around 18–22 weeks, and glucose screening tests in the second or early third trimester).
• Work and parental leave. Knowing your approximate third-trimester dates can help you and your partner plan leave from work, arrange coverage, and coordinate with employers or HR departments.
• Travel and events. Airlines, cruise lines, and some employers have policies about flying or certain activities later in pregnancy. Understanding when you will be in your third trimester can guide safer travel planning.
• Family logistics. The milestone schedule can help with arranging childcare for older children, support from relatives, or postpartum help at home.

This calculator focuses on dates and timing. For help planning nutrition, weight gain, or weekly changes during pregnancy, you can pair it with other tools on this site, such as a pregnancy calorie needs tool, a pregnancy weight gain tracker, and a week-by-week pregnancy overview.

Special situations and when this calculator may not apply

LMP-based dating works best when:

• Your periods are fairly regular.
• You remember the first day of your last menstrual period.
• You have not recently changed birth control methods or had hormonal treatments that disrupted your cycle.

In other situations, this calculator can give only a rough approximation or may not be appropriate at all. Examples include:

• Irregular or very long/short cycles. If your cycles vary widely or are often longer than 35 days or shorter than 21 days, ovulation timing may not follow the usual pattern.
• Uncertain LMP. If you are not sure when your last period started, the estimate may be off by days or weeks.
• Recent pregnancy loss or postpartum cycles. Ovulation can be unpredictable after a miscarriage, termination, or recent birth, especially in the first few cycles.
• Breastfeeding. Lactation can delay or disrupt ovulation; the first postpartum ovulation may occur before your first period, making LMP-based dating less reliable.
• Hormonal contraception changes. Coming off pills, patches, implants, or IUDs can temporarily change cycle patterns.
• Assisted reproductive technology (IVF, ICSI, IUI, ovulation induction). In these cases, clinics usually calculate due dates from the egg retrieval date, embryo transfer date, or a precisely known ovulation date, not just the LMP.
• Certain medical conditions. Conditions such as polycystic ovary syndrome (PCOS), thyroid disorders, and others can influence cycle regularity and may call for ultrasound-based dating.

If any of these apply to you, consider this calculator a general educational tool only. Always rely on your own healthcare team for your official gestational age and due date.

Limitations, assumptions, and safety notes

Because pregnancy and fetal development are complex, every due date estimate comes with important limitations. This calculator:

• Assumes a single pregnancy. Twins or higher-order multiples often arrive earlier than the 40-week mark, even when dates are calculated correctly.
• Uses a standard 280-day model. While common in obstetrics, some pregnancies naturally last a little shorter or longer without any clear medical problem.
• Relies on the LMP date you enter. Any error in recalling or entering that date will directly affect the estimate.
• Does not account for individual risk factors. Maternal age, prior births, medical conditions, and pregnancy complications can all influence if or when a provider recommends delivering earlier than the estimated due date.

This tool is for general education and planning only. It is not a medical device, diagnosis, or treatment plan, and it does not replace care from a qualified professional. Only your midwife, obstetrician, family doctor, or other prenatal care provider can confirm your baby’s gestational age and official due date.

Seek urgent medical care or call your local emergency number if you are pregnant and notice warning signs such as severe abdominal pain, heavy bleeding, sudden loss of fetal movement after feeling regular movements, fluid leaking from the vagina, severe headache with vision changes, chest pain, or difficulty breathing.

Common questions about due dates

Can my due date change?

Yes. Providers may revise your due date if early ultrasound measurements differ meaningfully from your LMP-based estimate, if your cycles are irregular, or if new information comes up about when you likely conceived. The updated clinical due date, if your provider offers one, should take priority over any calculator output.

What if I do not remember my last period?

If you do not know your LMP, this calculator can only provide a rough guess. In that case, contact a prenatal care provider. They may use ultrasound, exam findings, and your history to estimate gestational age more accurately.

Is it normal to give birth before or after 40 weeks?

Yes. Many babies are born in the two to three weeks on either side of the estimated due date. Delivering a bit earlier or later can be normal, although your provider will monitor you more closely if pregnancy continues beyond about 41 weeks or if you go into labor much earlier than expected.

Does the calculator tell me when I conceived?

The conception window shown is an estimate based on typical ovulation timing. Sperm can survive for several days, and ovulation can vary, so it is rarely possible to pinpoint the exact day of conception from an online calculator alone.

Who created this calculator?

This calculator is based on standard obstetric dating conventions that are widely described in clinical guidelines and textbooks. It is intended as a supportive planning aid. For any personal medical questions or decisions, always speak directly with your own healthcare team.