Pregnancy Calculator

Calculate your due date and track your pregnancy progress week by week.

What Is a Pregnancy Calculator?

A pregnancy calculator estimates your due date and tracks weekly progress based on either the first day of your last menstrual period or your known conception date. Using Naegele's rule as its foundation, the tool adds 280 days (40 weeks) from the LMP to produce an estimated delivery date. It then calculates your current gestational week, identifies your trimester, and provides milestone information about your baby's development at each stage.

Pregnancy is measured in gestational weeks rather than calendar months because medical professionals need a standardized timeline for scheduling prenatal appointments, tests, and screenings. The gestational clock starts on the first day of the last menstrual period, which means the first two weeks of pregnancy occur before conception actually takes place. This convention exists because the LMP date is typically known with more certainty than the ovulation or conception date.

How the Calculation Works

The standard method for estimating a due date is Naegele's rule, named after German obstetrician Franz Naegele. The formula is straightforward: take the first day of the last menstrual period, add one year, subtract three months, and add seven days. Mathematically, this is equivalent to adding 280 days to the LMP date.

For women with cycle lengths other than 28 days, the calculator applies an adjustment. It adds the difference between the actual cycle length and 28 to the standard 280-day duration. A woman with a 32-day cycle would have an adjusted due date of 284 days from her LMP, since ovulation likely occurred four days later than in a standard cycle.

When using the conception date method, the calculator adds 266 days (38 weeks) from the known conception date to estimate the due date. It also works backward to establish an equivalent LMP date for gestational age tracking by subtracting 14 days from the conception date.

The trimester breakdown divides the 40-week pregnancy into three periods. The first trimester covers weeks 1 through 12, the second trimester spans weeks 13 through 26, and the third trimester runs from week 27 until delivery. These divisions correspond to distinct phases of fetal development and maternal changes.

How to Use This Calculator

Choose your calculation method. Select either Last Menstrual Period or Conception Date. The LMP method is the standard clinical approach and works best if you have regular cycles. Choose the conception date method if you know the exact date of conception through fertility tracking or assisted reproduction.
Enter the relevant date. For the LMP method, select the first day of your most recent menstrual period from the date picker. For the conception method, select the date you believe conception occurred.
Adjust your cycle length. If using the LMP method, enter your average menstrual cycle length in days. The default is 28 days. If your cycles are consistently longer or shorter, adjusting this value improves accuracy.
Review your results. The calculator displays your current gestational age in weeks and days, your trimester, a visual progress bar, estimated due date, days remaining, current developmental milestone, baby's approximate size, and key milestone dates throughout the pregnancy.

Worked Examples

Example 1: Standard LMP Calculation

A woman whose last period began on September 1, 2025, with a 28-day cycle length would have a due date of June 8, 2026 (September 1 + 280 days). If checked on February 19, 2026, she would be at approximately 24 weeks and 2 days, nearing the end of the second trimester.

Example 2: Adjusted Cycle Length

A woman with a 35-day cycle whose LMP was August 15, 2025 would have an adjusted due date of May 29, 2026 (280 + 7 extra days = 287 days from LMP). The seven-day adjustment accounts for the later ovulation in a longer cycle.

Example 3: Conception Date Method

A couple who conceived through IVF on October 10, 2025 would calculate 266 days forward, yielding a due date of July 3, 2026. The equivalent gestational start date would be September 26, 2025 (14 days before conception).

Health Guidelines and Prenatal Care

Prenatal care typically follows a schedule tied to gestational age. During the first trimester, visits are usually monthly and include initial blood work, ultrasound dating, and genetic screening options. The second trimester involves the anatomy scan around week 20, glucose screening between weeks 24 and 28, and continued monthly visits. Third-trimester visits increase to biweekly and then weekly as the due date approaches, focusing on fetal position, growth, and signs of labor.

The American College of Obstetricians and Gynecologists recommends specific screenings at defined gestational intervals. First-trimester screening for chromosomal conditions occurs between weeks 11 and 14. The anatomy ultrasound is typically performed between weeks 18 and 22. Group B streptococcus screening happens between weeks 36 and 37.

Weight gain during pregnancy should follow the guidelines established by the Institute of Medicine, which are based on pre-pregnancy BMI. Normal-weight women are advised to gain 25 to 35 pounds, while underweight women should aim for 28 to 40 pounds, overweight women for 15 to 25 pounds, and obese women for 11 to 20 pounds.

Tips for Accurate Date Tracking

Record your LMP date as soon as your period begins. Relying on memory weeks or months later introduces error. Use a period tracking app or calendar to log the first day of each cycle consistently.

Understand that due dates are estimates, not guarantees. Normal full-term delivery can occur anywhere from 37 to 42 weeks. Think of the due date as the center of a delivery window rather than a fixed appointment.

Confirm your dates with a first-trimester ultrasound. Crown-rump length measurement between weeks 8 and 12 provides the most accurate dating and may adjust your due date by several days. If the ultrasound date differs from the LMP date by more than seven days, providers typically use the ultrasound date.

Track your cycle length before conception if possible. Several months of cycle data gives a reliable average that improves the accuracy of LMP-based calculations. Short cycles and long cycles shift the ovulation day and therefore the true conception window.

Do not change your due date based on subsequent ultrasounds. After the first trimester, ultrasound measurements reflect individual growth variation rather than gestational age differences. Later ultrasounds should not override an established due date.

Frequently Asked Questions

How accurate is a pregnancy due date calculator?

Due date calculators based on the last menstrual period use Naegele's rule, which adds 280 days to the first day of your LMP. This method is accurate to within about two weeks for women with regular 28-day cycles. Ultrasound dating in the first trimester is more precise, typically accurate to within five to seven days. Only about 5% of babies arrive on the exact calculated due date.

What is the difference between gestational age and fetal age?

Gestational age counts from the first day of your last menstrual period, even though conception typically occurs about two weeks later. Fetal age, also called embryonic age, counts from the actual date of conception. Medical professionals use gestational age as the standard because the LMP date is usually more certain than the conception date. Gestational age is always about two weeks longer than fetal age.

When does each trimester begin and end?

The first trimester spans weeks 1 through 12 and covers implantation and early organ formation. The second trimester runs from week 13 through week 26, during which the baby grows rapidly and movements become noticeable. The third trimester begins at week 27 and continues until birth around week 40. Some providers place trimester boundaries slightly differently.

Can irregular cycles affect my due date calculation?

Yes, irregular cycles can shift the estimated due date significantly. The standard calculation assumes a 28-day cycle with ovulation on day 14. If your cycles are longer or shorter, ovulation occurs on a different day, which changes the true conception date. Entering your average cycle length in the calculator adjusts the due date accordingly. First-trimester ultrasound provides the most reliable dating for irregular cycles.

What milestones should I watch for during pregnancy?

Key milestones include the first heartbeat detection around week 6, the end of the first trimester at week 12 when miscarriage risk drops significantly, the anatomy scan around week 20, viability at week 24 when survival outside the womb becomes possible with medical support, and full term at week 37 when the baby is considered ready for birth.

Is the conception date method more accurate than the LMP method?

If you know your exact conception date, such as through IVF or tracked ovulation, that method can be more accurate because it removes the uncertainty of ovulation timing. However, most people do not know their exact conception date. The LMP method works well for women with regular cycles and is the default approach used by obstetricians for initial dating before ultrasound confirmation.

How does cycle length affect the due date?

In a standard 28-day cycle, ovulation occurs around day 14. For longer cycles, ovulation happens later, pushing the due date further out. For shorter cycles, ovulation occurs earlier, moving the due date closer. The calculator adjusts by adding or subtracting the difference between your cycle length and 28 days. A 35-day cycle would shift the due date about one week later than a 28-day cycle.

What does post-term pregnancy mean?

A pregnancy that extends beyond 42 weeks of gestation is considered post-term. This occurs in roughly 5 to 10 percent of pregnancies. Post-term pregnancies carry increased risks including decreased amniotic fluid, larger birth weight, and reduced placental function. Most healthcare providers recommend induction of labor between 41 and 42 weeks to reduce these risks, though individual management plans vary.