Pregnancy Due Date Calculator

• Last Menstrual Period (LMP)
• Known Due Date

First Day of Last Menstrual Period Estimated Due Date

days days

Pregnancy Timeline: Weekly Milestones

Pregnancy Due Date Calculation

Expectant parents often eagerly anticipate their baby’s arrival. This calculator helps estimate the baby’s due date, which is typically 40 weeks (280 days) from the first day of the last menstrual period (LMP). While it provides a guide, the actual birth date can vary, as full-term babies can arrive between 37 and 42 weeks. [3, 5]

How Pregnancy is Dated

Pregnancy duration is usually counted from the first day of the woman’s last menstrual period, not the date of conception. This is because ovulation and conception typically occur about two weeks after the LMP. Healthcare providers commonly use the LMP to estimate a baby’s due date. [1, 19]

Naegele’s Rule

Naegele’s Rule is a widely used method to estimate the due date. It involves adding one year, subtracting three months, and adding seven days to the first day of the Last Menstrual Period (LMP). For example, if your LMP was January 1st, the due date would be October 8th of the following year. This rule assumes a regular 28-day menstrual cycle. [14]

Adjustments for Cycle Length

For individuals with menstrual cycles longer or shorter than 28 days, adjustments to the standard 280-day calculation are often made. If your cycle is longer than 28 days, the due date may be slightly later, and if shorter, it might be earlier. The calculator incorporates your average cycle length and luteal phase length to provide a more precise estimate of your conception date and due date. [14]

Estimated Conception Date

While the due date is calculated from the LMP, the actual conception date occurs later, typically around two weeks after the LMP in a standard 28-day cycle. If the precise date of conception is known, such as from assisted reproductive technologies, the due date can be calculated by adding 266 days (38 weeks) to the conception date. [14, 19]

Pregnancy Trimesters

Pregnancy is divided into three trimesters, each lasting about three months or 13-14 weeks. [1, 3, 4]

• First Trimester (Weeks 1-13): This period begins with the first day of the LMP and extends through week 13. It’s a time of rapid fetal development where all major organs and body systems begin to form. [1, 4] Common early pregnancy symptoms like missed periods, nausea, and fatigue are often experienced during this phase. [1]
• Second Trimester (Weeks 14-27): Many individuals find this trimester more comfortable as morning sickness often eases and energy levels may increase. [1] During this time, fetal movement may be felt, and the baby continues to grow significantly. [3]
• Third Trimester (Weeks 28-40+): This is the final stage, leading up to delivery. The baby undergoes significant growth and maturation, gaining weight and preparing for birth. [1]

Factors Influencing Due Date Accuracy

The estimated due date is a guide, not an exact prediction. Several factors can influence the accuracy of the calculation: [1]

• Accuracy of LMP: An accurate recall of the first day of the last menstrual period is crucial.
• Irregular Cycles: Women with irregular menstrual cycles may find LMP-based calculations less accurate.
• Ultrasound Dating: Early ultrasounds can provide a more precise estimated due date, especially when there’s uncertainty about the LMP. Ultrasound dating uses the size of the fetus to determine gestational age. [17]
• Individual Variation: Pregnancy duration can vary. A baby born between 37 weeks and 42 weeks is considered full-term. [3]