Calculate your due date from your last period, conception date, or IVF transfer. See your trimester breakdown and key pregnancy milestones.
A pregnancy due date is an estimate — not a guarantee of when your baby will arrive. Most babies are born between 38 and 42 weeks of pregnancy. Our pregnancy calculator uses Naegele's rule adjusted for your cycle length to give you the most accurate estimate possible from home. For an official due date, your healthcare provider will use a combination of your LMP and early ultrasound measurements. All calculations run entirely in your browser — your data never leaves your device.
A pregnancy due date is typically calculated using Naegele's Rule, formulated by German obstetrician Franz Karl Naegele in the early 19th century. The formula adds 280 days (40 weeks) to the first day of the last menstrual period (LMP). In practice: take the LMP date, add one year, subtract three months, and add seven days.
The 40-week count starts from the LMP — not from conception — because conception typically occurs approximately 14 days after the start of the period (for a standard 28-day cycle), and LMP is nearly always known while conception date is rarely certain. This means the first two weeks of the 40-week "pregnancy" technically predate fertilization.
Our calculator supports four calculation methods:
Pregnancy is divided into three trimesters:
Only about 4–5% of babies are born on their calculated due date. Approximately 80% of births occur between 38 and 42 weeks of gestation. A first-trimester ultrasound remains the most accurate method of confirming gestational age and establishing a clinical due date.
LMP: January 15 | Cycle length: 28 days
Estimated due date: January 15 + 280 days = October 22
Key milestones from this LMP:
If this person's cycle were 35 days instead of 28, ovulation would occur around day 21 rather than day 14 — and the due date would shift 7 days later to October 29. This cycle-length adjustment is one of the key refinements our calculator applies over the basic Naegele's rule.
Cycle length variation: Naegele's rule assumes a 28-day cycle with ovulation on day 14. Women with cycles significantly longer or shorter than 28 days will have due dates that differ from the standard calculation. Our calculator adjusts by the difference between actual cycle length and 28 days.
LMP vs. conception dating: LMP-based dating works well when cycles are regular. However, if a woman has irregular cycles, recently stopped hormonal birth control (which can delay ovulation), or experienced stress-delayed ovulation, the LMP date may not reliably reflect actual conception timing. In these cases, conception date (if known) or early ultrasound dating is more accurate.
Ultrasound dating: A first-trimester ultrasound between 8–13 weeks is considered the gold standard for confirming gestational age. Crown-rump length (CRL) measurement at this stage is accurate to within 5–7 days. If the ultrasound date differs significantly from the LMP estimate, providers typically use the ultrasound date as the official due date.
Multiple pregnancy: Twins, triplets, and higher-order multiples are typically delivered earlier than singleton pregnancies — often at 36–38 weeks for twins, 32–34 weeks for triplets. The same due date formula applies, but expectation management for delivery timing is different.
Preterm and post-term ranges: Preterm birth is defined as delivery before 37 weeks; late preterm (34–37 weeks) babies usually do well with minimal NICU intervention. Post-term pregnancy (beyond 42 weeks) increases the risk of certain complications, and most providers recommend induction if pregnancy continues past 41–42 weeks.
A pregnancy due date is calculated by adding 280 days (40 weeks) to the first day of your last menstrual period (LMP). This is the standard method used by most healthcare providers. If you know your conception date, the calculator adds 266 days (38 weeks) instead. For IVF, it uses the transfer date plus an adjustment for embryo age.
Naegele's rule is the standard method for calculating a due date: add 1 year, subtract 3 months, and add 7 days to the first day of the last menstrual period — which is equivalent to adding 280 days. It assumes a 28-day cycle with ovulation on day 14. Our calculator adjusts for different cycle lengths.
Due date calculators based on LMP are accurate to within a few days for women with regular 28-day cycles. However, only about 4–5% of babies are born on their exact due date. Most births occur within two weeks before or after the estimated due date. An ultrasound in the first trimester is the most accurate way to confirm your due date.
The first trimester runs from weeks 1–13, the second trimester from weeks 14–26, and the third trimester from weeks 27–40. Each trimester marks important developmental milestones. Our pregnancy calculator shows the exact date ranges for each trimester based on your due date.
Women with longer or shorter cycles ovulate at different times, which shifts the conception date and therefore the due date. Our calculator accounts for cycle length — if you have a 35-day cycle, it adjusts your due date by 7 days later than the standard calculation. Enter your actual cycle length for the most accurate estimate.