Due Date & Milestones
Last reviewed: May 2026
A pregnancy due date calculator estimates your delivery date, current gestational week, and key developmental milestones based on the first day of your last menstrual period (LMP). The standard calculation adds 280 days (40 weeks) to the LMP — known as Naegele's rule. Only about 5% of babies arrive on their exact due date, but roughly 80% are born within two weeks of it. This calculator helps you understand where you are in your pregnancy timeline and what to expect at each stage.1
Pregnancy is dated from the first day of the LMP, not from conception — which typically occurs about two weeks later during ovulation. This means a woman is already considered "two weeks pregnant" at conception and "four weeks pregnant" when she first misses a period. If a first-trimester ultrasound (most accurate between 8–13 weeks) shows a gestational age differing from the LMP-based estimate by more than 5–7 days, most providers adjust the due date to match the ultrasound. Later ultrasounds are less accurate for dating.2
| Trimester | Weeks | Key Developments | Common Symptoms |
|---|---|---|---|
| First | 1–12 | All major organs form, heartbeat detectable at 6 weeks | Nausea, fatigue, breast tenderness |
| Second | 13–26 | Fastest growth, movements felt, anatomy scan at 18–20 wk | Energy returns, round ligament pain |
| Third | 27–40 | Brain develops, lungs mature, gains ~0.5 lb/week | Back pain, swelling, Braxton Hicks |
| Week | Milestone |
|---|---|
| 6–8 | First prenatal visit, heartbeat confirmation |
| 10–13 | Optional genetic screening (NIPT, NT scan) |
| 18–20 | Anatomy ultrasound — detailed structural check |
| 24 | Viability milestone — survival possible with NICU care |
| 24–28 | Glucose tolerance test for gestational diabetes |
| 36 | Group B strep screening, weekly visits begin |
| 37 | Early term — baby is considered mature |
| 39–40 | Full term — ideal delivery window |
The American College of Obstetricians and Gynecologists defines term pregnancies as: Early term (37w0d–38w6d), Full term (39w0d–40w6d), Late term (41w0d–41w6d), and Post-term (42w0d+). Babies born at full term (39–40 weeks) have the best outcomes. Elective deliveries before 39 weeks without medical indication increase respiratory and developmental risks. First-time mothers tend to deliver slightly later than their due date on average.3
The Institute of Medicine recommends weight gain based on pre-pregnancy BMI: underweight (BMI <18.5): 28–40 lbs; normal weight (18.5–24.9): 25–35 lbs; overweight (25–29.9): 15–25 lbs; obese (30+): 11–20 lbs. Most gain occurs in the second and third trimesters. Use our Pregnancy Weight Gain Calculator to track against guidelines.4
Pregnancy due dates are estimated using Naegele's Rule, developed 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), which is equivalent to adding one year, subtracting three months, and adding seven days. For example, if the LMP was March 1, the estimated due date would be December 8. This calculation assumes a 28-day menstrual cycle with ovulation on day 14 — assumptions that do not apply to all women, which is why first-trimester ultrasound dating has become the preferred method for establishing accurate due dates when cycle lengths are irregular.
Alternative calculation methods include conception-based dating (adding 266 days from known conception date), IVF transfer dating (adding 266 days minus embryo age at transfer), and ultrasound dating using crown-rump length measurement in the first trimester. First-trimester ultrasound dating is considered the most accurate method, with a margin of error of approximately ±3-5 days when performed between 8-13 weeks. When ultrasound and LMP dates differ by more than 5-7 days, most providers adopt the ultrasound date. Despite all methods, only about 4-5% of babies arrive on their exact due date — the "due date" is better understood as the center of a probability distribution spanning several weeks.
Two different aging systems are used during pregnancy, which can cause confusion. Gestational age counts from the first day of the LMP — this is the standard system used in medical records, prenatal care schedules, and when someone says they are "X weeks pregnant." Fetal age (also called conceptional age or embryonic age) counts from the actual date of fertilization, which is approximately two weeks after the LMP for women with regular 28-day cycles. A woman who is "8 weeks pregnant" by gestational age has a fetus that is approximately 6 weeks old by fetal age.
This distinction matters when interpreting developmental milestones and test results. Medical literature, prenatal screening timelines, and viability assessments all use gestational age. Embryology textbooks and some research papers use fetal age. Ultrasound measurements in early pregnancy determine gestational age based on standardized growth charts — crown-rump length, biparietal diameter, and femur length measurements are compared to reference populations to estimate gestational age with increasing accuracy as more measurement parameters become available. For week-by-week developmental details, our Pregnancy Week Calculator provides comprehensive milestone information.
The first trimester (weeks 1-12) is the period of organogenesis — all major organ systems are forming, making this the most critical time for developmental sensitivity to medications, toxins, and nutritional deficiencies. The heart begins beating at approximately week 5-6. By week 8, the embryo has recognizable human features and is reclassified as a fetus. First-trimester screening (weeks 11-14) assesses risk for chromosomal conditions. Nausea peaks around weeks 8-10 and typically improves by weeks 12-14. Miscarriage risk drops significantly after a heartbeat is confirmed (to approximately 2-4% after week 8 with confirmed heartbeat).
The second trimester (weeks 13-27) is often called the "golden period" as early symptoms subside while the baby is not yet large enough to cause significant physical discomfort. The anatomy scan at weeks 18-22 is a detailed ultrasound evaluating fetal structures, placental location, and amniotic fluid. Fetal movement is typically first felt between weeks 16-22 (earlier for subsequent pregnancies). Viability — the gestational age at which survival outside the womb becomes possible with intensive care — is reached at approximately 22-24 weeks, though outcomes improve dramatically with each additional week through 28 weeks.
The third trimester (weeks 28-40+) is characterized by rapid fetal growth and preparation for delivery. The fetus gains approximately half a pound per week. Lung maturation is a critical developmental process during this period — surfactant production, which prevents the lungs from collapsing between breaths, reaches adequate levels around weeks 34-36. Brain development accelerates dramatically, with the brain's surface area increasing and neural connections multiplying rapidly. Most babies assume a head-down position by weeks 32-36 in preparation for birth.
Several factors influence when a baby actually arrives relative to the estimated due date. First-time mothers (nulliparas) tend to deliver slightly later than the estimated date — on average 5-7 days past the due date, with approximately 50% of first births occurring after 40 weeks and 5 days. Mothers who have previously given birth (multiparas) tend to deliver closer to or slightly before the due date. Maternal age, ethnicity, and genetic factors also influence gestational length — some studies suggest that Black women have a slightly shorter average gestational period than white women, which has implications for clinical management decisions based on gestational age.
Environmental and lifestyle factors play roles as well. Stress, physical activity levels, nutritional status, and maternal health conditions (gestational diabetes, preeclampsia, hypertension) can all influence timing of delivery. Medical interventions including induction of labor and scheduled cesarean sections also affect actual delivery dates — current guidelines recommend against elective delivery before 39 weeks due to the continued developmental benefits of late gestation, particularly for lung maturation and brain development. Understanding that the "due date" represents a statistical estimate rather than a precise prediction helps manage expectations and reduces anxiety as the date approaches and passes. For financial planning around your baby's arrival, our Baby Cost Calculator and Childcare Cost Calculator help prepare for the costs ahead.
Pregnancy dating uses several methods with varying accuracy. Last menstrual period (LMP) dating calculates the due date as 280 days (40 weeks) from the first day of the last menstrual period, assuming a 28-day cycle with ovulation on day 14. This method can be inaccurate for women with irregular cycles, longer or shorter cycle lengths, or uncertain LMP dates. First-trimester ultrasound dating (performed between 6-13 weeks) measures the crown-rump length of the embryo and is considered the most accurate dating method, with a margin of error of only ±5-7 days. When ultrasound dating disagrees with LMP dating by more than 7 days in the first trimester (or more than 10-14 days in the second trimester), the ultrasound date is generally used. Conception dating, when known (from IVF or tracked ovulation), is the most precise method — the due date is 266 days (38 weeks) from conception. Understanding which dating method was used is important because the estimated due date affects scheduling of prenatal tests, assessment of fetal growth, and decisions about labor induction.
→ Your due date is an estimate. Only 5% of babies arrive on the exact date. Think of it as the center of a 4-week window (38–42 weeks).
→ First-trimester ultrasound is most accurate. If available, an early ultrasound (8–13 weeks) provides the most reliable due date estimate.
→ Track weight gain by trimester. Most gain occurs in the second and third trimesters. Use our Pregnancy Weight Gain Calculator to stay within recommended ranges.
→ Know your term classifications. Full term is 39–40 weeks. Early term (37–38) is safe but slightly less optimal. Discuss timing of any planned deliveries with your provider.
See also: Due Date Calculator · Ovulation Calculator · Pregnancy Weight Gain · Baby Cost Calculator