Pregnancy Due Date Calculator

Your due date is calculated using Naegele's Rule, which adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). The formula works by taking the first day of your LMP, subtracting 3 months, and then adding 7 days plus one year. This method assumes a 28-day menstrual cycle with ovulation occurring on day 14. This is the standard method used by healthcare providers, though it's important to remember that only about 4-5% of women deliver exactly on their due date - most deliver within two weeks before or after.

Pregnancy is divided into three trimesters, each approximately 13-14 weeks long: First Trimester (Weeks 1-12): Early development of the embryo, formation of major organs, and when morning sickness typically occurs. Second Trimester (Weeks 13-26): Often called the 'golden period' with less nausea, noticeable baby movement, and gender determination possible. Third Trimester (Weeks 27-40+): Rapid baby growth, preparation for birth, and increasing physical discomfort as the baby takes up more room. Each trimester has distinct developmental milestones and different considerations for maternal health.

Ultrasound accuracy for dating pregnancy depends on when it's performed. Early ultrasounds (before 14 weeks) are most accurate, with a margin of error of about 5-7 days. First-trimester ultrasounds, especially between 11-14 weeks, are considered the gold standard for dating pregnancy with accuracy within 5 days. Second-trimester scans (14-26 weeks) have a margin of error of about 10-14 days. Third-trimester ultrasounds are the least accurate for dating, with a margin of error of 2-3 weeks. If your early ultrasound date differs significantly from your LMP date, healthcare providers typically use the ultrasound date as it's usually more accurate, especially for women with irregular cycles.

Several factors can affect the accuracy of due date calculations: 1) Irregular menstrual cycles make LMP-based calculations less reliable, as ovulation may not occur on day 14, 2) Recent hormonal birth control use can affect cycle regularity, 3) Breastfeeding or recent pregnancy can delay ovulation, 4) Maternal age - younger women often deliver closer to their due date, while older first-time mothers more commonly deliver later, 5) Previous pregnancy history - your labor pattern may repeat, 6) Genetic factors from both parents, and 7) Multiple pregnancies typically deliver earlier (37 weeks is full-term for twins). These variables explain why due dates are considered estimates rather than firm deadlines.

Gestational age and fetal age are two different ways of measuring pregnancy duration. Gestational age is calculated from the first day of your last menstrual period (LMP) and is typically about two weeks longer than fetal age. This is the standard clinical measurement used by healthcare providers and is what people mean when they say someone is '12 weeks pregnant.' Fetal age (or embryonic age) measures the actual time since conception/fertilization, which typically occurs about two weeks after the LMP in a 28-day cycle. For example, a fetus with a gestational age of 10 weeks has a fetal age of approximately 8 weeks. Medical literature and prenatal care use gestational age for consistency.

Only about 4-5% of women deliver exactly on their due date. Pregnancy is considered full-term between 37 and 42 weeks, and most spontaneous deliveries occur within this five-week window. Statistics show that: 1) About 50% of first-time mothers deliver after their due date, 2) About 70% deliver within 10 days of their due date (either before or after), 3) About 90% deliver within two weeks of their due date. Women who have given birth before are slightly more likely to deliver earlier than first-time mothers. Due dates are best viewed as the middle of a range rather than an exact delivery date.

According to the American College of Obstetricians and Gynecologists (ACOG), pregnancy terms are defined as: 1) Early Term: 37 weeks, 0 days through 38 weeks, 6 days, 2) Full Term: 39 weeks, 0 days through 40 weeks, 6 days, 3) Late Term: 41 weeks, 0 days through 41 weeks, 6 days, and 4) Post Term: 42 weeks, 0 days and beyond. These distinctions matter because babies born during the full-term period typically have the best health outcomes. Babies born before 39 weeks may face slightly higher risks of breathing problems, feeding difficulties, and temperature regulation issues, while post-term pregnancies can present increased risks for both mother and baby.

If you know your exact conception date (from fertility tracking, assisted reproduction, or a single instance of intercourse), you can calculate a potentially more accurate due date by adding 266 days (38 weeks) to the conception date. This is because pregnancy actually lasts about 38 weeks from conception, rather than 40 weeks from LMP. Some healthcare providers will adjust your due date if the conception date is definitively known, particularly in cases of IVF where the embryo transfer date is precisely documented. However, many providers still use LMP-based dating for consistency in medical records, even when conception date is known, and may only adjust if there's a significant discrepancy with ultrasound findings.

As you approach your due date, several signs may indicate labor is approaching: 1) Lightening/dropping - when the baby descends lower into the pelvis, creating more breathing room but increasing pressure on the bladder, 2) Increased Braxton Hicks contractions that may become more regular, 3) Cervical changes including effacement (thinning) and dilation (opening), 4) Loss of the mucus plug and/or bloody show, 5) Rupture of membranes (water breaking), 6) Loose stools or diarrhea, 7) Sudden energy burst or nesting instinct, 8) Lower back pain or cramping that doesn't go away with position changes. True labor typically involves contractions that increase in frequency, duration, and intensity, while false labor contractions (Braxton Hicks) are irregular and often subside with rest or hydration.

Research shows that maternal age can influence pregnancy duration. Women over 35, particularly first-time mothers (nulliparous women), tend to have slightly longer pregnancies on average. Studies indicate that women over 40 are more likely to go beyond 41 weeks compared to younger women. Conversely, very young mothers (teens) sometimes have higher rates of preterm delivery. However, these are statistical trends with many individual variations. Other factors like race/ethnicity also play a role - studies show that pregnancy duration varies slightly among different ethnic groups. Your healthcare provider considers multiple factors, not just age, when monitoring your pregnancy progress and making recommendations about timing of delivery.