Pregnancy Due Date Calculator

How Is Your Pregnancy Due Date Calculated?

Your estimated due date (EDD) is typically calculated using one of three methods depending on the information available. This calculator supports all three so you can choose the approach that best matches your situation.

Naegele's Rule (LMP Method)

The most common method for calculating a due date is Naegele's rule, developed by German obstetrician Franz Karl Naegele in the early 19th century. The formula is straightforward: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a standard 28-day cycle where ovulation occurs on day 14.

For cycles that differ from 28 days, the calculator adjusts accordingly. If your cycle is 30 days, for example, ovulation likely occurs around day 16 rather than day 14, shifting your due date two days later. This cycle-length adjustment is built into this calculator automatically.

IVF Transfer Date Method

For pregnancies achieved through in vitro fertilization (IVF), the due date is calculated from the embryo transfer date. With a day-5 blastocyst transfer, 261 days are added to the transfer date, because the embryo is already 5 days old at the time of transfer and the total gestational period is 266 days from fertilization (plus 14 days for the LMP convention = 280 days). For a day-3 cleavage-stage transfer, 263 days are added.

IVF due dates are generally considered more accurate than LMP-based dates because the exact age of the embryo is known.

Conception Date Method

If you know your approximate conception date (e.g., from tracking ovulation), 266 days (38 weeks) are added to arrive at the due date. Conception typically occurs about 14 days after the LMP in a 28-day cycle, which is why there is a 14-day difference between the LMP and conception-date methods.

Understanding Gestational Age

Gestational age counts pregnancy in weeks and days from the first day of the LMP — not from conception. This convention is used universally in obstetrics because the LMP date is usually known, while the exact conception date often is not. At the moment of conception, you are already considered approximately 2 weeks pregnant by gestational age.

Healthcare providers use gestational age to schedule prenatal appointments, order screening tests at the appropriate times, and assess fetal growth. Ultrasound measurements in the first trimester can refine gestational age estimates and may be used to adjust the due date.

The Three Trimesters

Pregnancy is traditionally divided into three trimesters of roughly 13 weeks each:

• First trimester (weeks 1-13): Organ formation begins. Morning sickness, fatigue, and breast tenderness are common. The nuchal translucency (NT) scan at around 11-13 weeks screens for chromosomal conditions.
• Second trimester (weeks 14-27): Often called the "honeymoon trimester" as many discomforts ease. The anatomy scan at 18-22 weeks checks fetal development in detail. Fetal movements become noticeable around 18-22 weeks.
• Third trimester (weeks 28-40+): Rapid growth and preparation for birth. The lungs mature, and the baby moves into head-down position. Braxton Hicks contractions may be felt. The cervix begins to prepare for labor.

Key Milestone Dates Explained

This calculator tracks seven clinically significant milestones throughout pregnancy:

• 12 weeks — NT scan: The nuchal translucency ultrasound screens for chromosomal abnormalities such as Down syndrome. Combined with a blood test, it forms part of first-trimester screening.
• 20 weeks — Anatomy scan: A detailed ultrasound examines the baby's brain, heart, spine, kidneys, limbs, and other structures. The placenta position and amniotic fluid are also assessed.
• 24 weeks — Viability: The threshold at which a premature baby may survive outside the womb with intensive neonatal support. Survival rates improve significantly with each additional week of gestation.
• 28 weeks — Third trimester begins: The third trimester starts. Prenatal visits typically increase to every two weeks, and kick-count monitoring may be recommended.
• 37 weeks — Full term: The baby is considered full term and the lungs are typically mature enough for independent breathing. Planned deliveries before this point are avoided unless medically indicated.
• 40 weeks — Due date: Your estimated due date. Only about 5% of babies are born exactly on this date, but roughly 80% arrive within two weeks on either side.
• 42 weeks — Post-term: A pregnancy that extends beyond 42 weeks is considered post-term. Healthcare providers typically recommend induction or closer monitoring after 41 weeks due to increased risks.

Week-by-Week Baby Size

A popular way to visualize fetal growth is to compare the baby's approximate size to familiar fruits and vegetables. At 6 weeks, the embryo is about the size of a sweet pea. By 12 weeks, it has grown to roughly the size of a lime. At 20 weeks, the baby is about the length of a banana. By 40 weeks, a full-term baby is approximately the size of a small watermelon, typically measuring 19-21 inches (48-53 cm) and weighing 6-9 pounds (2.7-4.1 kg).

These comparisons are approximate guides. Actual fetal size varies based on genetics, nutrition, and other factors. Ultrasound measurements provide a more accurate assessment of individual fetal growth.