Pregnancy Due Date Calculator – Estimate Gestational Age
Free online pregnancy due date calculator. Find your estimated due date.
Finding out you are pregnant brings a flood of questions, and one of the very first is: when is my due date? A pregnancy due date calculator estimates the expected delivery date based on the first day of your last menstrual period (LMP) or the date of conception. While only about 5% of babies arrive on their exact due date, having a target date is essential for scheduling prenatal appointments, planning work leave, preparing for delivery, and tracking fetal development milestones throughout the 40-week gestational period.
Key Takeaway
The most common method for calculating a due date is Naegele's rule: subtract 3 months from the first day of the last menstrual period, then add 7 days and 1 year. This gives an estimated due date of 280 days (40 weeks) from the LMP. Ultrasound dating in the first trimester provides the most accurate confirmation of gestational age.
How Due Date Calculation Works
Pregnancy dating begins with the first day of the last menstrual period (LMP) rather than the date of conception. This convention exists because the exact date of conception is rarely known, while the first day of the last period is typically identifiable. Obstetricians use the LMP because it provides a consistent starting point that aligns with the beginning of the menstrual cycle, even though actual conception usually occurs about two weeks later during ovulation.
Naegele's rule, named after the 19th-century German obstetrician Franz Karl Naegele, is the standard clinical method. The formula works as follows: take the first day of the last menstrual period, subtract 3 calendar months, then add 7 days, and finally add 1 year. For example, if the LMP was January 15, 2026, subtracting 3 months gives October 15, adding 7 days gives October 22, and adding 1 year gives October 22, 2027. This result corresponds to 280 days or 40 weeks of gestation.
While 40 weeks is the standard gestational age at delivery, normal pregnancies can range from 37 to 42 weeks. Babies born before 37 weeks are considered preterm, while those born after 42 weeks are considered post-term. The due date serves as a midpoint estimate rather than a precise deadline.
Ultrasound Confirmation and Accuracy
While LMP-based dating is the initial standard, ultrasound provides the most accurate determination of gestational age, particularly when performed in the first trimester. Early ultrasounds measure the crown-rump length (CRL) of the embryo, which correlates very precisely with gestational age up to about 13 weeks. First-trimester ultrasounds typically confirm or adjust the LMP-based due date by a few days.
After the first trimester, ultrasound accuracy for dating decreases as fetal growth rates become more variable. If an early ultrasound confirms the LMP-based due date, subsequent ultrasounds are generally used to monitor fetal growth rather than to recalculate the due date. In cases where the LMP is uncertain, irregular periods make dating unreliable, or the ultrasound dating differs significantly from the LMP date, the due date is typically adjusted to match the ultrasound estimate.
For women undergoing in vitro fertilization (IVF), the due date calculation is more precise because the exact date of embryo transfer is known. The gestational age is calculated by adding the number of days since embryo transfer to the developmental age of the embryo at transfer, then computing the expected delivery date from that known gestational age.
Tracking Trimesters and Fetal Development
Pregnancy is divided into three trimesters, each with distinct developmental milestones. The first trimester (weeks 1 through 13) encompasses the most critical period of organ formation. The heart begins beating around week 6, major organs start developing by week 8, and by the end of the first trimester, the fetus has formed all basic organs and limbs. This is also when morning sickness is typically most severe.
The second trimester (weeks 14 through 27) is often considered the most comfortable period. Morning sickness typically subsides, energy levels increase, and the baby grows rapidly. By week 20, the baby is approximately the size of a banana, and parents can often discover the sex through ultrasound. Fetal movement, called quickening, usually begins between weeks 18 and 25.
The third trimester (weeks 28 through 40) involves the final stages of lung development, rapid weight gain, and preparation for delivery. The baby's brain develops rapidly, fat stores increase for temperature regulation after birth, and the baby typically moves into a head-down position in preparation for birth. Understanding where you are in this timeline helps you prepare both physically and emotionally for each stage of pregnancy, and our due date calculator gives you the reference point for tracking all of these milestones.