Pregnancy Due Date Calculator

Calculate estimated delivery date and pregnancy milestones. Track trimesters, important prenatal dates, and fetal development stages.

First Day of Last Menstrual Period

About Pregnancy Due Date Calculator

Understanding Due Date Calculation

Our pregnancy due date calculator uses Naegele's rule to estimate your baby's arrival date based on your last menstrual period (LMP). This method, developed by German obstetrician Franz Naegele in the early 1800s, adds 280 days (40 weeks) to the first day of your last period. While simple, it remains surprisingly accurate for many pregnancies and is still the standard starting point for pregnancy dating today.

Understanding Pregnancy Dating

The 40-week pregnancy countdown actually begins about two weeks before conception typically occurs. This might seem counterintuitive, but it's based on the average menstrual cycle where ovulation occurs around day 14. When doctors talk about how far along you are, they're referring to "gestational age" (counted from LMP), not "fetal age" (actual time since conception). This standardized approach helps establish consistent prenatal care timing regardless of when conception actually occurred.

The Three Trimesters

First Trimester (Weeks 1-12)

Often called the "foundation stage" because all major organ systems begin forming. During these crucial early weeks, your baby's neural tube develops into the brain and spinal cord, the heart begins to beat, and tiny limb buds form. Though you may not look pregnant yet, your body is undergoing significant hormonal changes. Many women experience morning sickness, fatigue, and breast tenderness. This is also when pregnancy risks are highest, making early prenatal care especially important.

Second Trimester (Weeks 13-26)

Often considered the "golden period" of pregnancy when many women feel their best. Morning sickness typically subsides, energy levels increase, and the risk of miscarriage drops significantly. Your baby grows from about 3 inches to 14 inches long during this time, develops facial features you can recognize on ultrasound, and begins moving noticeably (quickening). Around week 20, the anatomy scan ultrasound can reveal your baby's sex if you wish to know. This trimester is also when many women begin showing a visible pregnancy bump.

Third Trimester (Weeks 27-40)

The final stretch focuses on rapid growth and maturation of your baby's organs. Your baby gains significant weight, develops lung function, and assumes the head-down birth position (in most cases). You may experience Braxton Hicks contractions, increased back pain, and difficulty sleeping. Regular prenatal appointments monitor your baby's position, growth, and your health for complications like gestational diabetes or preeclampsia. By week 37, your pregnancy is considered "full term," meaning your baby's systems are ready for life outside the womb.

Key Pregnancy Milestones

Early Pregnancy (Weeks 1-12)

Week 4: Implantation occurs, placenta begins forming
Week 8: All major organs begin forming, heartbeat detectable
Week 12: Baby becomes a fetus, risk of miscarriage drops

Mid-Pregnancy (Weeks 13-27)

Week 16: Baby's movements may be felt
Week 20: Anatomy scan, gender can often be determined
Week 24: Viability milestone reached

Late Pregnancy (Weeks 28-40)

Week 28: Third trimester begins, rapid brain growth
Week 32: Baby practicing breathing movements
Week 37: Full term begins
Week 40: Due date

Factors Affecting Due Date Accuracy

Cycle Variations

Irregular Cycles: Naegele's rule assumes a 28-day cycle
Late Ovulation: Can make due date appear off by up to two weeks
Cycle Length: Adjust for cycles longer or shorter than 28 days

Other Factors

Ultrasound Dating: More accurate in early pregnancy
Assisted Reproduction: IVF dates are more precise
Previous Pregnancies: May affect delivery timing

Planning Around Your Due Date

Your estimated due date helps you and your healthcare provider plan important aspects of your pregnancy journey. Remember that only about 5% of babies arrive exactly on their due date, with:

50% of births occurring within one week of the due date (before or after)
75% of births occurring within two weeks of the due date
First babies are more likely to arrive later than subsequent babies
Most healthcare providers consider induction after 41 weeks

Frequently Asked Questions

How is a due date calculated?

A due date is typically calculated using Naegele's rule, which adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). This method assumes a regular 28-day menstrual cycle and ovulation occurring on day 14. However, only about 4% of pregnancies actually deliver on their exact due date - it's normal for birth to occur up to two weeks before or after this date.

Why do doctors count pregnancy from the last period instead of conception?

Doctors start counting pregnancy weeks from the first day of your last menstrual period (LMP) because it's a more reliable date to track from than conception. While conception typically occurs about two weeks after your period starts, the exact date of conception is often unclear. Using LMP as a starting point provides a standardized way to measure pregnancy progress, even though you aren't actually pregnant during those first two weeks.

What affects the accuracy of a due date calculation?

Several factors can affect due date accuracy: irregular menstrual cycles can make LMP-based calculations less reliable; late ovulation can lead to later conception than assumed; and remembering the exact date of your last period can be difficult. For more accurate dating, doctors often use ultrasound measurements in the first trimester, which can be accurate within 3-5 days when performed early in pregnancy.

Learn More

ACOG Fetal Development Guide NICHD Pregnancy Resources WHO Pregnancy Guidelines

Important Note:

This calculator provides an estimate based on statistical averages. Your actual due date may differ based on individual factors. Always consult with your healthcare provider for personalized pregnancy care and information.

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