Find your estimated due date and track every pregnancy milestone from first prenatal visit to delivery day. Enter your last menstrual period, conception date, IVF transfer date, or ultrasound measurements — the calculator adjusts automatically for cycle length and dating method.
Pro tip: Only about 5% of babies arrive on their estimated due date. Most are born within two weeks on either side, so think of your EDD as the centre of a delivery window rather than a fixed appointment.
How to Use the Due Date Calculator
Start by choosing your preferred dating method. The most common approach is Last Menstrual Period (LMP) — select the first day of your most recent period and adjust the cycle length if yours differs from the standard 28 days. If you know the exact day of conception, switch to Conception Date mode. Patients who conceived through assisted reproduction can use the IVF Transfer option and specify whether a Day 3 or Day 5 embryo was transferred. Finally, the Ultrasound mode lets you enter the scan date along with the gestational age your technician measured in weeks and days. Whichever method you choose, the calculator instantly displays your estimated due date, current gestational age, trimester, a pregnancy progress bar, key prenatal appointment dates, and a visual 40-week milestone timeline.
Understanding Naegele’s Rule
The standard formula behind most due date calculations is Naegele’s Rule, published by German obstetrician Franz Naegele in 1812. The rule estimates a full-term pregnancy at 280 days (40 weeks) from the first day of the last menstrual period. Because it assumes ovulation on day 14 of a 28-day cycle, the formula can be adjusted for women with longer or shorter cycles by adding or subtracting the difference from 28. For example, a 32-day cycle shifts the due date four days later, while a 24-day cycle brings it four days earlier. Despite its simplicity, Naegele’s Rule remains the clinical standard and matches ultrasound dating within a few days for the majority of pregnancies when cycle length is accurately reported.
Trimester Breakdown and What to Expect
Pregnancy is divided into three trimesters of roughly 13 weeks each. The first trimester (weeks 1–13) covers implantation, organogenesis, and the formation of every major body system — by week 12 the fetus has a beating heart, developing limbs, and fingerprints. The second trimester (weeks 14–27) is often called the “honeymoon period” because many uncomfortable first-trimester symptoms fade; this is also when the anatomy scan takes place and parents often learn the sex. The third trimester (weeks 28–40) is marked by rapid weight gain, lung maturation, and the baby settling into a head-down position in preparation for delivery. Understanding which trimester you are in helps you anticipate changes, prepare for upcoming tests, and communicate with your healthcare team.
Why So Few Babies Arrive on Their Due Date
Research consistently shows that only about 5% of babies are born on their estimated due date. The reason is biological variability — gestation length naturally spans a range of about five weeks (37 to 42 weeks) among healthy pregnancies. Factors such as the mother’s age, whether she has given birth before, genetics, and even the length of her own gestation as a baby all influence timing. First-time mothers tend to deliver slightly later than experienced mothers. Because of this spread, obstetricians consider any delivery between 39 weeks and 40 weeks 6 days to be “full term” and discourage elective interventions before 39 weeks unless medically indicated. Thinking of your EDD as the midpoint of a window rather than a deadline can help manage expectations as the date approaches.
How Cycle Length Affects Your Due Date
The standard 28-day cycle assumes ovulation around day 14, but healthy cycles can range from 21 to 35 days. Women with longer cycles typically ovulate later, meaning conception happens later relative to the start of the last period. If Naegele’s Rule is applied without adjustment, these women receive a due date that is too early — potentially leading to unnecessary concern about a “late” baby. Our calculator adds the difference between your actual cycle length and 28 days to the standard 280-day calculation. A 35-day cycle adds 7 days; a 21-day cycle subtracts 7 days. If you are unsure of your average cycle length, the 28-day default is statistically the most common and provides a reasonable starting estimate. Your provider may refine the date with a first-trimester ultrasound regardless.
IVF and Ultrasound Dating Methods
For pregnancies conceived through in vitro fertilisation, the dating is more precise because the exact fertilisation and transfer dates are known. A Day 5 blastocyst transfer means the embryo is already five days past fertilisation at the time of transfer, so the due date is calculated as the transfer date plus 261 days (280 minus 14 days to account for the two-week pre-ovulation phase, plus the embryo’s age adjustment). A Day 3 transfer adds 263 days instead. Ultrasound dating works differently: the sonographer measures crown-rump length in the first trimester or head circumference later and assigns a gestational age. The calculator subtracts that age from the scan date to find an estimated LMP, then adds 280 days. First-trimester ultrasounds are accurate to within plus or minus five days, making them the gold standard when the LMP is uncertain or the cycle length is irregular.
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