We use cookies to improve your experience. By continuing to use this site, you agree to our use of cookies.
Calculate your estimated due date and track your pregnancy milestones. Enter your last menstrual period or conception date.
Due dates are most commonly calculated using Naegele's rule, which adds 280 days (40 weeks) to the first day of your last menstrual period. This method assumes a regular 28-day cycle with ovulation occurring on day 14. If you know your conception date, the due date is 266 days (38 weeks) from that date.
Pregnancy is divided into three trimesters. The first trimester (weeks 1-13) involves rapid cell division, organ formation, and early development. The second trimester (weeks 14-26) brings visible growth, movement, and often relief from early symptoms. The third trimester (weeks 27-40) focuses on weight gain, lung maturation, and preparation for delivery.
Key milestones include the first heartbeat detection around week 6, the first ultrasound at week 8, the anatomy scan at week 20, the viability milestone at week 24, and reaching full term at week 37. Each milestone marks an important stage in your baby's development.
Regular prenatal visits are essential for monitoring both mother and baby's health. Typical schedules include monthly visits until week 28, bi-weekly visits from weeks 28-36, and weekly visits from week 36 until delivery. These visits include check-ups, screenings, and opportunities to discuss any concerns with your healthcare provider.
Pregnancy dating is rooted in the biology of human reproduction and embryonic development. From a scientific perspective, the gestational period of 280 days (40 weeks) is measured from the first day of the last menstrual period (LMP), even though conception typically occurs approximately two weeks later during ovulation. This convention exists because the LMP date is usually known with certainty, while the exact date of conception is often uncertain. The actual embryonic age (also called fertilization age) is therefore about two weeks less than the gestational age. After fertilization, the embryo undergoes rapid cell division, implanting in the uterine wall around day 6-10. By week 5 (gestational age), the embryonic heart begins to form, and a heartbeat can typically be detected by week 6-7 via transvaginal ultrasound. The first trimester is the most critical period for organ development (organogenesis), during which all major organ systems are established. This is why exposure to teratogenic substances is most dangerous in the first 12 weeks. Modern research has refined our understanding of fetal development timelines. A large-scale study published in the journal Human Reproduction found that the length of pregnancy can naturally vary by as much as 37 days among healthy women. Factors such as maternal age, BMI, previous pregnancy history, and the timing of embryo implantation all influence gestation length. This natural variation is why only about 5% of babies are born on their calculated due date, and why clinicians consider full-term to be a range from 39 weeks 0 days to 40 weeks 6 days.
The most widely used method for calculating a due date is Naegele's rule, named after German obstetrician Franz Karl Naegele (1778-1851). The rule is straightforward: take the first day of the last menstrual period, subtract three months, then add seven days and one year. For example, if the LMP is March 20, subtracting three months gives December 20, then adding seven days gives December 27 — the estimated due date. This rule assumes a 28-day menstrual cycle with ovulation on day 14. For women with cycles shorter or longer than 28 days, the due date can be adjusted by adding or subtracting the difference. Parikh's formula offers an alternative: LMP + 9 months - 21 days + duration of previous cycles. This method accounts for individual cycle variation and may be more accurate for women with irregular cycles. Ultrasound dating, particularly when performed in the first trimester (before 13 weeks 6 days), is considered the most accurate method with a margin of error of plus or minus 5-7 days. It uses crown-rump length (CRL) measurements to estimate gestational age. Second-trimester ultrasounds are accurate to within plus or minus 7-14 days, while third-trimester estimates have a margin of plus or minus 21-30 days. When there is a discrepancy of more than 7 days between the LMP-based date and first-trimester ultrasound, the American College of Obstetricians and Gynecologists (ACOG) recommends using the ultrasound date.
First Trimester (Weeks 1-13): Begin taking 400-800 mcg of folic acid daily (ideally before conception) to prevent neural tube defects. Eat frequent small meals to manage nausea, and stay hydrated. Avoid alcohol, smoking, and raw or undercooked meats. Schedule your first prenatal visit between weeks 6-8. Get recommended blood work including blood type, Rh factor, and infection screening. Moderate exercise like walking and swimming is safe and beneficial for most women. Second Trimester (Weeks 14-26): This is often called the 'golden trimester' as nausea usually subsides and energy returns. Schedule your anatomy scan around week 18-22 to check for structural abnormalities and potentially learn the baby's sex. Begin sleeping on your left side to improve blood flow to the uterus. Consider prenatal classes for labor preparation. Maintain a balanced diet with increased calcium (1,000 mg/day) and iron (27 mg/day). Watch for warning signs such as persistent headaches, visual changes, or sudden swelling. Third Trimester (Weeks 27-40): Monitor fetal movement daily — you should feel at least 10 movements within 2 hours during active periods. Prepare your hospital bag by week 36. Attend weekly prenatal appointments from week 36. Learn the signs of preterm labor: regular contractions before 37 weeks, lower back pressure, increased vaginal discharge, or fluid leakage. Practice perineal massage from week 34 to reduce the risk of tearing during delivery. Discuss your birth plan with your healthcare provider, including pain management preferences.
While most pregnancies progress normally, it is important to be aware of risk factors and warning signs that require medical attention. Advanced maternal age (over 35) is associated with increased risk of chromosomal abnormalities, gestational diabetes, preeclampsia, and cesarean delivery. Pre-existing conditions including diabetes, hypertension, thyroid disorders, and autoimmune diseases require careful management during pregnancy. Gestational diabetes affects 6-9% of pregnancies and typically develops between weeks 24-28, which is why glucose screening is recommended during this period. Left unmanaged, it can lead to excessive fetal growth (macrosomia), birth complications, and increased future diabetes risk for both mother and child. Preeclampsia, characterized by high blood pressure and protein in urine after week 20, affects 5-8% of pregnancies and can progress to life-threatening eclampsia if untreated. Warning signs include severe headaches, visual disturbances, upper abdominal pain, sudden swelling of face and hands, and significant blood pressure elevation. Seek immediate medical attention for vaginal bleeding, severe abdominal pain, fever over 38 degrees C (100.4 degrees F), sudden decrease in fetal movement, fluid leaking from the vagina before 37 weeks, or persistent severe headaches with visual changes. Regular prenatal care is the most effective way to identify and manage complications early, significantly improving outcomes for both mother and baby.
Pregnancy due date estimation is primarily based on Naegele's rule, named after German obstetrician Franz Karl Naegele (1778-1851). The rule adds 280 days (40 weeks) to the first day of the last menstrual period (LMP), assuming a regular 28-day cycle with ovulation occurring on day 14. For women who know their conception date, the due date is calculated by adding 266 days (38 weeks) from that date. The two-week difference accounts for the interval between the start of the menstrual period and ovulation, which is why gestational age is always about two weeks more than the actual embryonic age.
Pregnancy is divided into three trimesters, each with distinct developmental milestones. The first trimester (weeks 1-13) is the most critical period for organ development. By week 5, the embryonic heart begins to form; by week 8, all major organ systems are established; and by week 12, the fetus is fully formed in miniature with functioning kidneys, a developing brain, and the ability to make spontaneous movements. The second trimester (weeks 14-26) brings rapid growth and maturation: the fetus develops fingerprints, can hear sounds, and begins the characteristic sleep-wake cycles that parents often notice as periods of activity.
The third trimester (weeks 27-40) is focused on weight gain, lung maturation, and preparation for life outside the womb. The lungs produce surfactant, a substance essential for breathing air, beginning around week 26 but not reaching adequate levels until approximately week 34-36. This is a key reason why premature births before 34 weeks carry significant respiratory risks. By week 37, the pregnancy is considered early term, with full term defined as 39 weeks 0 days through 40 weeks 6 days.
Ultrasound dating provides the most accurate due date estimation, particularly when performed in the first trimester. Crown-rump length measurement between 8-13 weeks has a margin of error of only 5-7 days, compared to 7-14 days for second-trimester ultrasounds and 21-30 days for third-trimester estimates. When there is a discrepancy of more than 7 days between the LMP-based date and a first-trimester ultrasound, the American College of Obstetricians and Gynecologists recommends using the ultrasound date. Notably, only about 5% of babies are born on their exact due date, with most arriving within a two-week window around the estimated date.
The primary calculation uses Naegele's rule: take the first day of the last menstrual period, subtract three months, then add seven days and one year. For example, if the LMP is March 20, subtracting three months gives December 20, then adding seven days yields an estimated due date of December 27. When using a conception date, the calculator adds 266 days (38 weeks) directly. For cycle lengths other than 28 days, the calculator adjusts by adding or subtracting the difference from ovulation day 14.
The trimester boundaries are calculated as follows: the first trimester spans weeks 1 through 13 (days 1-91), the second trimester covers weeks 14 through 26 (days 92-182), and the third trimester extends from week 27 through week 40 (days 183-280). Key milestone dates are derived from the estimated due date by counting backward: the viability milestone at 24 weeks, the anatomy scan window at 18-22 weeks, and the first-trimester screening window at 11-14 weeks. The gestational age display is calculated by finding the difference between the current date and the LMP date, expressed in completed weeks and days.