👶 Pregnancy Due Date Calculator

Understanding Your Estimated Due Date

Your estimated due date (EDD) is calculated by adding 280 days (40 weeks) to the first day of your last menstrual period (LMP). This standard method, known as Naegele's Rule, was developed in the 19th century and remains the universal starting point for pregnancy dating. The 280-day figure assumes a 28-day menstrual cycle with ovulation on day 14, making conception occur approximately 2 weeks after the LMP date — which is why gestational age counts from LMP, not from the moment of conception.

Only about 4–5% of babies are born on their exact due date. Approximately 80% of births occur within 2 weeks before or after the EDD. A full-term pregnancy is considered 39–40 weeks; 37–38 weeks is early term; 41–42 weeks is late term. Your healthcare provider will discuss the appropriate monitoring and possible interventions if pregnancy extends beyond 41 weeks.

The Three Trimesters of Pregnancy

First Trimester (Weeks 1–12):

• Weeks 1–2: Technically pre-conception; the clock starts from LMP.
• Weeks 3–4: Fertilization and implantation occur. hCG hormone rises — detectable by pregnancy test.
• Weeks 5–10: All major organ systems begin forming (embryonic period). Heart begins beating around week 6. Critical period when organs are most vulnerable to developmental disruptions.
• Weeks 10–12: The embryo becomes a fetus. All essential structures are present. Nuchal translucency ultrasound typically performed at 11–13 weeks.
• Common symptoms: Nausea/morning sickness, fatigue, breast tenderness, frequent urination, heightened sense of smell.

Second Trimester (Weeks 13–27):

• Often called the "golden trimester" — nausea typically subsides, energy returns, and the risk of miscarriage drops significantly.
• Weeks 16–20: Most women feel fetal movement ("quickening") for the first time.
• Week 18–22: Anatomy ultrasound performed to check development and can reveal sex if desired.
• Weeks 24–28: Glucose tolerance test screens for gestational diabetes. Baby's viability outside the womb begins around 24 weeks with intensive care.
• Baby's length grows from about 3 inches to 14 inches during this trimester.

Third Trimester (Weeks 28–40):

• Rapid weight gain and growth — the baby gains most of its birth weight during this period.
• Lungs mature and brain development accelerates significantly.
• Common symptoms: Back pain, frequent urination, shortness of breath, Braxton Hicks contractions, difficulty sleeping.
• Weeks 35–36: Group B Streptococcus (GBS) test typically performed.
• Week 36+: Prenatal appointments increase to weekly as birth approaches.
• Weeks 37–39: Baby "drops" into the pelvis (lightening), often providing breathing relief but increasing pelvic pressure.

Dating Methods Used by Healthcare Providers

• LMP Method (this calculator): The standard first estimate. Accuracy depends on having regular, 28-day cycles. Can be off by 1–2 weeks with irregular cycles.
• First Trimester Ultrasound (most accurate): Crown-rump length measurement at 6–13 weeks is accurate to ±3–5 days. If ultrasound dating differs from LMP by more than 5–7 days in the first trimester, providers typically revise the due date to match the ultrasound.
• Second Trimester Ultrasound: Accurate to ±1–2 weeks at 14–20 weeks. Less precise than first trimester but still useful if no early ultrasound was performed.

Key Prenatal Appointments and Milestones

• 8–10 weeks: First prenatal visit, initial blood work, confirm pregnancy and EDD
• 11–13 weeks: First trimester screening (nuchal translucency ultrasound + blood tests)
• 15–20 weeks: Optional quad screen or NIPT (non-invasive prenatal testing)
• 18–22 weeks: Anatomy scan ultrasound
• 24–28 weeks: Glucose challenge test for gestational diabetes
• 28 weeks: Rh-negative mothers receive Rh immunoglobulin injection
• 35–36 weeks: GBS swab; increasing appointment frequency begins
• 36–40 weeks: Weekly check-ups; cervical assessment; birth plan discussions

Frequently Asked Questions

What if my cycles aren't 28 days long? Naegele's Rule assumes a 28-day cycle. If your cycle is longer (e.g., 35 days), you likely ovulated later, and your due date may be pushed back by the difference. An early ultrasound gives a more reliable date than LMP for women with irregular or longer cycles. Your provider will reconcile these methods at your first appointment.

What does it mean if my baby is "measuring ahead" or "behind"? Ultrasound measurements of the baby's size are compared to typical growth curves for gestational age. Small variations (1–2 weeks off) are common and usually not concerning. Consistent measurements significantly outside the expected range may prompt additional monitoring to assess growth and amniotic fluid levels.

Important notice: This calculator provides an estimate only and is not a substitute for professional medical care. Always work with your obstetrician, midwife, or healthcare provider for personalized prenatal guidance, accurate dating, and monitoring throughout your pregnancy.