Due Date Calculator | Pregnancy Due Date
Calculate your estimated due date using 5 methods: Last Menstrual Period (LMP), conception date, IVF transfer (Day 3 or Day 5), or ultrasound dating. Adjusts for cycle length. See also Pregnancy Calculator and Ovulation Calculator.
Normal range: 21-35 days (default 28)
How is the Due Date Calculated?
Your estimated due date (EDD) is calculated using Naegele's Rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, the calculator adjusts by adding or subtracting the difference from 14 days. Only about 5% of babies are born on their exact due date — most arrive within 2 weeks before or after.
Due Date Formulas by Method
LMP Method: Due Date = LMP + 280 days + (Cycle Length - 28)
Conception: Due Date = Conception Date + 266 days
IVF Day 3: Due Date = Transfer Date + 263 days
IVF Day 5: Due Date = Transfer Date + 261 days
Ultrasound: Due Date = Scan Date + (280 - Gestational Age in days)
Which Method is Most Accurate?
First Trimester Ultrasound (Most Accurate)
Accurate to within 3-5 days when performed between 7-13 weeks. Measures crown-rump length (CRL). If the ultrasound date differs from LMP by more than 7 days, most providers will adjust the due date.
IVF Transfer Date (Very Accurate)
Since the exact fertilization date is known, IVF due dates are highly accurate. Day 5 blastocyst transfers are slightly more precise than Day 3 transfers.
LMP with Known Cycle Length (Good)
Accurate to within 7-10 days for women with regular cycles. Less accurate for irregular cycles. Adjusting for cycle length improves accuracy significantly.
Conception Date (Good if Known)
Accurate if you know the exact date of intercourse that led to conception. However, sperm can survive up to 5 days, so the actual fertilization date may differ from intercourse date.
Pregnancy Timeline Reference
| Week | Development | Baby Size |
|---|---|---|
| 4 | Implantation, positive test possible | Poppy seed |
| 8 | Heart beating, limbs forming | Raspberry |
| 12 | All organs formed, end of 1st trimester | Lime |
| 16 | Gender visible on ultrasound | Avocado |
| 20 | Anatomy scan, movements felt | Banana |
| 24 | Viability milestone, lungs developing | Corn on the cob |
| 28 | Eyes open, brain developing rapidly | Eggplant |
| 32 | Bones hardening, gaining weight fast | Squash |
| 36 | Lungs nearly mature, head down | Honeydew melon |
| 40 | Full term, ready for birth | Watermelon |
Frequently Asked Questions
How accurate is a due date calculator?
Only about 5% of babies arrive on their exact due date. About 80% are born within 2 weeks of the estimated date (between 38-42 weeks). First trimester ultrasound is the most accurate dating method (within 3-5 days). LMP-based calculations are accurate to within 7-10 days for regular cycles.
What if my cycle is irregular?
If your cycles vary significantly (more than 7 days between cycles), LMP-based dating is less reliable. In this case, a first trimester ultrasound (7-13 weeks) provides the most accurate due date. You can also use the conception date method if you tracked ovulation with OPKs or basal body temperature.
Can my due date change?
Yes. If a first trimester ultrasound shows a gestational age that differs from LMP dating by more than 7 days, most providers will adjust the due date to match the ultrasound. Later ultrasounds are less accurate for dating (within 2-3 weeks in the third trimester) and are not used to change the due date.
What is the difference between gestational age and fetal age?
Gestational age counts from the first day of your last period (LMP) — this is what doctors use and what "40 weeks pregnant" refers to. Fetal age (embryonic age) counts from conception, which is approximately 2 weeks less. A baby at 40 weeks gestational age is actually about 38 weeks old from conception.
What does "full term" mean?
Full term is defined as 39 weeks 0 days through 40 weeks 6 days. Early term is 37-38 weeks, late term is 41 weeks, and post-term is 42+ weeks. Babies born at 39+ weeks have the best outcomes, which is why elective inductions and C-sections are not recommended before 39 weeks without medical indication.