Due Date Calculator
Due Date Calculator
The Due Date Calculator estimates your baby's expected delivery date based on either your last menstrual period (LMP) or your conception date. Knowing your estimated due date helps you track pregnancy milestones, plan prenatal appointments, and prepare for the arrival of your baby. The estimated due date is the foundation of prenatal care scheduling and provides a timeline for monitoring fetal development throughout each trimester.
The standard method for calculating due dates is Naegele's rule, which adds 280 days (40 weeks) to the first day of the last menstrual period. [acog-due-date] This method assumes a regular 28-day menstrual cycle with ovulation occurring on day 14. When the conception date is known, such as through assisted reproductive technology or precise ovulation tracking, the due date can be calculated by adding 266 days (38 weeks) from conception, which accounts for the approximately two weeks between LMP and ovulation. In vitro fertilization (IVF) pregnancies may have even more precise dating based on embryo transfer date.
It is important to understand that the estimated due date is exactly what it sounds like: an estimate. Only about 5% of babies are born on their exact due date. Most pregnancies last between 37 and 42 weeks, with a full-term pregnancy considered to be between 39 and 40 weeks. The due date provides a general timeframe for the end of pregnancy rather than a precise prediction. Early ultrasound measurements, particularly in the first trimester, provide the most accurate dating and may adjust the due date calculated from LMP alone. First-trimester crown-rump length measurements are accurate to within 5-7 days.
The Due Date Calculator is straightforward to use. Select your preferred calculation method and enter the relevant date.
- Select Calculation Method — Choose whether to calculate based on your Last Menstrual Period (LMP) or your Conception Date. The LMP method is more commonly used because many women know when their last period started. The conception method is useful if you tracked ovulation or conceived through fertility treatments such as IUI or IVF.
- Enter the Date — Input the relevant date using the date picker. For the LMP method, enter the first day of your last menstrual period. For the conception method, enter the estimated date of conception (ovulation date). Be as accurate as possible for the best results.
- Review Your Due Date — The calculator displays your estimated due date and the number of days remaining. Use this information to plan your prenatal appointments, maternity leave, and preparations for your baby's arrival. Remember that this is an estimate and your actual delivery date may vary. The estimated due date is the foundation of prenatal care scheduling and provides a timeline for monitoring fetal development.
Naegele's Rule
The due date is calculated using Naegele's rule:
The 280-day figure represents 40 weeks of pregnancy, which is considered full term. The 266-day figure represents 38 weeks from conception to delivery, subtracting the approximately 14 days between LMP and ovulation from the total 280 days.
Example Calculations
| Method | Start Date | Calculation | Due Date |
|---|---|---|---|
| LMP | May 1, 2026 | May 1 + 280 days | February 4, 2027 |
| Conception | May 1, 2026 | May 1 + 266 days | January 21, 2027 |
Gestational Age Timeline
| Week | Milestone |
|---|---|
| 4 weeks | Missed period; pregnancy test positive |
| 8 weeks | First trimester ultrasound possible |
| 12 weeks | End of first trimester; NT scan |
| 20 weeks | Anatomy scan (mid-pregnancy ultrasound) |
| 24 weeks | Fetal viability |
| 28 weeks | Third trimester begins |
| 37 weeks | Early term; baby considered full term |
| 39-40 weeks | Full term; most babies born in this window |
| 41 weeks | Late term |
| 42 weeks | Post-term; induction often recommended |
Understanding how pregnancy progresses through each trimester helps you know what to expect and when key developmental milestones occur. The 40 weeks of pregnancy are divided into three trimesters, each with distinct changes for both the baby and the mother.
First Trimester (Weeks 1-12)
The first trimester is a period of rapid development and organ formation, known as organogenesis, which occurs primarily between weeks 3 and 10. By week 5, the embryonic heart begins to beat, and a heartbeat is typically detectable on ultrasound by week 6. The neural tube, which becomes the brain and spinal cord, completes formation by week 6. Arms, legs, fingers, and toes develop between weeks 6 and 10. By week 8, all major organs have begun forming, and by week 12, the fetus has developed most of its basic structures.
Prenatal care should begin early in the first trimester. The first prenatal visit usually occurs between weeks 8 and 12 and includes confirmation of pregnancy, blood type screening, Rh factor testing, complete blood count, urine analysis, and screening for infectious diseases. First-trimester screening for chromosomal abnormalities, including the nuchal translucency (NT) ultrasound and blood tests, is typically performed between weeks 11 and 13.
Miscarriage risk is highest during the first trimester, affecting an estimated 10-15% of known pregnancies. Most losses occur before week 12, often due to chromosomal abnormalities in the developing embryo. Early and consistent prenatal care helps monitor for potential complications and provides guidance on nutrition and lifestyle adjustments.
Common first-trimester symptoms include fatigue, morning sickness, breast tenderness, and frequent urination. Many women also experience food aversions, mood changes, and heightened sense of smell due to hormonal shifts. Despite these challenges, adequate rest, small frequent meals, and prenatal vitamins containing folic acid support healthy fetal development.
Second Trimester (Weeks 13-27)
The second trimester is often considered the most comfortable period of pregnancy. Morning sickness typically subsides, energy levels improve, and the pregnancy becomes more visible as the baby bump grows. This is also when many women begin to feel emotionally connected to the pregnancy.
A key milestone is the anatomy scan, performed between weeks 18 and 22. This detailed ultrasound examines the baby's organs, limbs, spine, heart, and brain, and can often reveal the sex if the parents wish to know. The technician measures head circumference, biparietal diameter, femur length, abdominal circumference, and checks placental position and amniotic fluid levels. Any abnormalities detected may prompt follow-up testing or specialist consultation.
Quickening, the first sensation of fetal movement, typically occurs between weeks 16 and 22. First-time mothers often feel movement later, while women who have been pregnant before may recognize it as early as 14 weeks. These early movements feel like flutters or gas bubbles and progress to distinct kicks, rolls, and stretches as the baby grows stronger.
Glucose screening for gestational diabetes is performed between weeks 24 and 28. This involves drinking a sweetened solution followed by a blood test one hour later. If the glucose level is elevated, a three-hour oral glucose tolerance test is used for diagnosis. Untreated gestational diabetes can lead to excessive fetal growth and delivery complications, making this screening an important part of prenatal care.
Third Trimester (Weeks 28-40)
The third trimester is marked by continued rapid fetal growth and preparation for delivery. Lung development accelerates significantly after week 28, with surfactant production increasing to prepare the lungs for breathing after birth. Babies born after week 32 have excellent survival rates with modern medical support. Fetal viability, the ability to survive outside the womb with intensive medical intervention, is generally considered to begin at 24 weeks, though outcomes improve significantly with each additional week in utero.
Braxton-Hicks contractions, sometimes called practice contractions, may begin in the third trimester. These irregular, typically painless contractions help prepare the uterus for labor. Unlike true labor contractions, they do not become stronger or closer together over time and often subside with rest, hydration, or changes in position. True labor contractions continue regardless of activity and progressively intensify.
During the final weeks, the baby typically settles into a head-down (cephalic) position in preparation for birth. This process, called engagement or lightening, often occurs a few weeks before labor begins. The healthcare provider checks the baby's presentation during prenatal visits. If the baby is breech (feet or buttocks first) after 36 weeks, the provider may discuss external cephalic version to turn the baby or plan for cesarean delivery if needed.
Prenatal appointments become more frequent in the third trimester, occurring every two weeks from weeks 28 to 36 and weekly after week 36. These visits include checking the baby's position, measuring fundal height, monitoring blood pressure, testing urine for protein, and screening for Group B Streptococcus between weeks 36 and 37. This timeline of increasing visit frequency ensures timely detection of complications such as preeclampsia, gestational hypertension, and fetal growth concerns.
First Trimester Dating Accuracy
Early pregnancy ultrasounds provide the most accurate dating:
| Gestational Age | Accuracy Range |
|---|---|
| First trimester (CRL) | 5-7 days |
| 14-20 weeks | 7-10 days |
| 20-26 weeks | 10-14 days |
| After 26 weeks | 14-21 days |
Due Dates by LMP
| LMP Date | Estimated Due Date |
|---|---|
| January 1 | October 7 |
| March 15 | December 19 |
| June 1 | March 7 (next year) |
| September 1 | June 7 (next year) |
| December 1 | September 6 (next year) |
Ultrasound dating is the most accurate method for estimating gestational age and due date, particularly when performed in the first trimester. While Naegele rule based on LMP provides a useful starting point, ultrasound measurements offer a more precise, individualized assessment of fetal development that accounts for variations in cycle length and ovulation timing.
Crown-Rump Length (CRL)
The crown-rump length measures the fetus from the top of the head (crown) to the bottom of the buttocks (rump), excluding the limbs. This measurement is most accurate between 8 and 14 weeks of gestation, with a precision of approximately plus or minus 5 to 7 days. During this window, fetal growth follows a remarkably consistent pattern across different populations, making CRL the gold standard for pregnancy dating.
After 14 weeks, the fetus begins to assume a curved, flexed position that makes CRL measurement less reliable. At this point, healthcare providers shift to a combination of other biometric measurements for dating and growth assessment. First-trimester ultrasound dating is so reliable that it is preferred over LMP-based dating whenever available, especially when the two methods differ by more than seven days.
Other Ultrasound Measurements
Later in pregnancy, dating accuracy decreases because fetal growth rates become more variable due to genetic factors, maternal nutrition, placental function, and the intrauterine environment. The key measurements used include:
Biparietal Diameter (BPD) — The transverse diameter across the fetal skull. This measurement is useful from approximately 13 weeks onward and correlates well with gestational age up to about 20 weeks. After 20 weeks, BPD becomes more useful for assessing growth patterns than for precise dating.
Femur Length (FL) — The length of the thigh bone provides a reliable indicator of fetal growth throughout pregnancy. Femur length correlates well with gestational age and is often combined with other measurements for a composite estimate of fetal size and age.
Abdominal Circumference (AC) — This measurement reflects liver size, stomach content, and subcutaneous fat. Abdominal circumference is the most variable of the standard measurements and is less reliable for dating but valuable for assessing fetal growth patterns and estimating birth weight.
Why First-Trimester Dating Is Most Reliable
In early pregnancy, fetal growth is primarily driven by gestational age itself rather than genetic or environmental factors. All healthy embryos grow at nearly the same rate during the first trimester, regardless of maternal age, ethnicity, or body composition. This consistency makes CRL dating accurate across diverse populations. After the first trimester, growth rates begin to diverge as genetics, nutrition, and placental sufficiency influence development. A fetus that measures large at 30 weeks may simply be genetically large rather than older than expected.
How Dating Scans Adjust the Due Date
When an early ultrasound produces a due date that differs from the LMP-based calculation, medical guidelines recommend using the ultrasound date if the discrepancy exceeds seven days in the first trimester. For second-trimester discrepancies, a 10- to 14-day difference may be required before adjusting, reflecting the reduced precision of later measurements. Most healthcare providers use the first ultrasound performed between 8 and 14 weeks as the basis for the official due date, provided the CRL measurement was technically adequate.
Multiple Pregnancy Dating
For twin and multiple pregnancies, accurate dating is critical because it affects decisions about delivery timing and monitoring for complications such as twin-to-twin transfusion syndrome and discordant growth. Dating for multiples relies on the same first-trimester CRL measurements used for singletons. The larger fetus is typically used for dating, as the smaller may be affected by early growth restriction. Multiple pregnancies deliver earlier on average, with mean gestational lengths of approximately 36 weeks for twins, 33 weeks for triplets, and 30 weeks for quadruplets.
In vitro fertilization pregnancies have the most precisely known conception dates of any pregnancy, making their due date calculations exceptionally accurate. The LMP-based approach does not apply to IVF pregnancies because the menstrual cycle is medically controlled and ovulation timing is bypassed through fertility treatment.
Fresh Embryo Transfer
For fresh embryo transfers, the due date depends on the developmental stage of the embryo at the time of transfer. A day 3 embryo transfer uses a baseline of 263 days from the transfer date, accounting for the three days of embryonic development that already occurred in the laboratory. A day 5 blastocyst transfer uses 261 days from the transfer date, since the embryo has developed for five days before being placed into the uterus. The general formula is: due date equals transfer date plus (266 minus embryo age in days). For a day 5 blastocyst, this is 266 minus 5 equals 261 days. For a day 3 embryo, this is 266 minus 3 equals 263 days.
Frozen Embryo Transfer (FET)
Frozen embryo transfer calculations depend on the type of cycle used. In natural cycle FET, the timing of ovulation is tracked and used similarly to natural conception dating. In medicated cycles using hormone replacement therapy, the endometrium is prepared with estrogen and progesterone, and the transfer date determines the due date using the same formula as fresh transfers. The embryo age at freezing, the number of days in culture after thawing, and the day of progesterone initiation all factor into the precise calculation. Clinics typically provide patients with a calculated due date based on the specific protocol used.
Why IVF Dating Is More Accurate
IVF due dates are considered the most reliable pregnancy dates for several reasons. The exact moment of fertilization is observed directly in the laboratory under controlled conditions. There is no ambiguity about ovulation timing or cycle length. The embryo age at transfer is precisely documented to the day. Studies consistently show that IVF-based dating has significantly less variability than LMP-based dating, with a margin of error of approximately plus or minus 3 days rather than 5 to 7 days for ultrasound dating.
Donor Egg and Surrogacy
For pregnancies using donor eggs, the due date is calculated based on the embryo transfer date to the recipient or surrogate, not the donor's menstrual cycle history. Similarly, in gestational surrogacy arrangements, the due date depends entirely on the embryo transfer date into the surrogate. This approach provides consistent results regardless of which individual contributed the egg or carried the pregnancy, and it is the standard practice in reproductive medicine.
Cycle Irregularity — Naegele's rule assumes a regular 28-day cycle with ovulation on day 14. Women with longer or shorter cycles, irregular cycles, or conditions like PCOS may ovulate at different times, making the LMP-based calculation less accurate. For these women, first-trimester ultrasound provides a more reliable due date.
Ultrasound Dating Discrepancy — Due dates calculated from LMP may differ from those determined by early ultrasound. When there is a discrepancy, ultrasound dating is generally considered more accurate, especially when performed between 8 and 13 weeks of pregnancy. Healthcare providers typically use ultrasound dating to confirm or adjust the LMP-based due date.
Not a Guarantee — The estimated due date is not a guarantee of when delivery will occur. Only about 5% of births happen on the exact due date. A normal pregnancy can range from 37 to 42 weeks. The calculator provides a reference point for planning purposes rather than a precise prediction.
Singleton Pregnancy Assumption — This calculator assumes a singleton pregnancy. Multiple pregnancies (twins, triplets) typically have shorter gestational lengths and different due date considerations. Women carrying multiples should rely on their healthcare provider's recommendations for due date estimation.
Schedule Your First Prenatal Visit — Once you receive your estimated due date, schedule your first prenatal appointment. Early prenatal care is associated with better pregnancy outcomes. Many healthcare providers schedule the first visit between 8 and 12 weeks of pregnancy.
Track Pregnancy Milestones — Use your due date to mark key pregnancy milestones on your calendar. Knowing when each trimester begins and when important screenings occur helps you stay organized and prepared throughout your pregnancy journey.
Prepare for Flexibility — While having a due date is helpful for planning, remain flexible about the actual delivery date. Pack your hospital bag by 36 weeks and have your birth plan ready, but understand that your baby may arrive earlier or later than expected.
Discuss with Your Healthcare Provider — Share your calculated due date with your healthcare provider, who may adjust it based on ultrasound measurements and clinical assessment. Your provider's dating is the official date used for your medical records and care planning.
Plan Your Maternity or Paternity Leave — Use your due date to coordinate your leave with your employer. Submit the necessary paperwork well in advance and discuss any flexible work arrangements, phased return options, or remote work possibilities. Having a clear plan reduces stress as your due date approaches and ensures you can focus on your family without work-related distractions.
Prepare Your Hospital Bag by 36 Weeks — Having your hospital bag packed and ready by 36 weeks prevents last-minute scrambling if labor begins unexpectedly. Essentials include comfortable clothing, toiletries, phone chargers, important documents, snacks for labor, and baby items such as a going-home outfit, diapers, wipes, and a receiving blanket. Keep the bag in a visible location so you can grab it on the way out the door.
What to Do If You Go Past Your Due Date — Only about 5% of babies arrive on their exact due date, and going past 40 weeks is common. At 41 weeks, your healthcare provider will discuss induction of labor, as the risks associated with post-term pregnancy begin to increase. After 42 weeks, induction is typically recommended because the placenta may become less efficient, and the risk of stillbirth rises. Additional monitoring such as non-stress tests, biophysical profiles, and amniotic fluid checks may be performed during this period.
Learn the Signs of Labor — True labor contractions become progressively longer, stronger, and closer together, and they do not subside with rest or position changes. Other signs include your water breaking (rupture of membranes), a bloody show (passing the mucus plug), and the baby dropping lower into the pelvis (lightening). Contact your healthcare provider when contractions are five minutes apart for at least one hour, or immediately if your water breaks or you experience significant vaginal bleeding.
- How does Naegele rule calculate my due date?
- Naegele rule estimates due date by adding 280 days (40 weeks) to the first day of your last menstrual period (LMP). Formula: LMP + 1 year - 3 months + 7 days.
- How accurate is the due date calculated by LMP versus conception date?
- Conception date calculation is more accurate because it does not rely on a 28-day cycle assumption. If you know ovulation or conception date, adding 266 days (38 weeks) gives a more precise estimate.
- How accurate is a due date calculation overall?
- Only about 4-5% of babies are born on their exact due date. The due date is an estimate of when you will be 40 weeks pregnant. Most babies arrive between 38 and 42 weeks, and a healthy pregnancy can range from 37 to 42 weeks.
- What percentage of babies are born on their exact due date?
- Only about 4-5% of babies are born on their exact due date. The due date is an estimate of when you will be 40 weeks pregnant. Most babies arrive between 38 and 42 weeks.
- Can irregular cycles affect my due date calculation?
- Yes. Naegele rule assumes a 28-day cycle with ovulation on day 14. For irregular cycles, an early ultrasound measurement (crown-rump length between 11-14 weeks) is the most accurate way to date a pregnancy.
- Why might my healthcare provider adjust my due date after an ultrasound?
- Ultrasound in the first trimester can date a pregnancy within 5-7 days of accuracy. If the ultrasound differs from LMP-based date by more than 7 days, providers typically adjust to the ultrasound date.
- What is the due date for twins or multiples?
- Multiple pregnancies typically deliver earlier than singletons. The average gestational length is about 36 weeks for twins and 32 weeks for triplets. Due date calculation for multiples uses the same first-trimester ultrasound dating methods, but delivery timing depends on medical considerations.
- What does full term mean?
- Full term is defined as 39 completed weeks to 40 weeks and 6 days. Early term is 37 weeks to 38 weeks and 6 days. Late term is 41 weeks to 41 weeks and 6 days. Post-term is 42 weeks and beyond. Full-term babies have the lowest rates of respiratory complications and other health issues.
- Can stress delay labor?
- There is limited evidence that maternal stress directly delays labor. While chronic stress may influence pregnancy outcomes through hormonal pathways, most women go into labor when their body and baby are ready. If you are past your due date and concerned, discuss induction options with your healthcare provider.
- What happens if I go past my due date?
- At 41 weeks, your provider may discuss induction options and recommend additional monitoring such as non-stress tests, biophysical profiles, and amniotic fluid assessments. After 42 weeks, induction is strongly recommended as the risk of stillbirth, placental insufficiency, and meconium aspiration syndrome increases with post-term pregnancy.
- [1]Baskett, T.F., & Nagele, F. (2000). Naegele's rule: a reappraisal. BJOG: An International Journal of Obstetrics and Gynaecology, 107(11), 1433-1435.
- [2]Committee on Obstetric Practice. (2017). Methods for Estimating the Due Date. ACOG Committee Opinion No. 700. Obstetrics and Gynecology, 129(5), e150-e154.
- [3]Savitz, D.A., Terry, J.W., Dole, N., Thorp, J.M., Siega-Riz, A.M., & Herring, A.H. (2002). Comparison of pregnancy dating by last menstrual period, ultrasound scanning, and their combination. American Journal of Obstetrics and Gynecology, 187(6), 1660-1666.
- [4]Middleton, P., Shepherd, E., & Crowther, C.A. (2017). Induction of labour for improving birth outcomes for women at or beyond term. Cochrane Database of Systematic Reviews, (5).
Last updated: July 8, 2026
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