Understanding Gestational Age and Pregnancy Weeks
Pregnancy duration is measured in gestational weeks, counted from the first day of your last menstrual period (LMP), even though conception typically occurs about 14 days later. A full-term pregnancy is 40 weeks (280 days) from LMP. The first trimester spans weeks 1–12, the second trimester weeks 13–26, and the third trimester weeks 27–40. Knowing your gestational age is important for prenatal care scheduling, interpreting ultrasound measurements, and understanding your baby's developmental milestones. This calculator uses the standard obstetric method based on LMP, or allows you to calculate from a known conception date.
What Is Gestational Age and How Is It Measured?
Gestational age refers to how far along a pregnancy is, measured in weeks and days from the first day of the mother's Last Menstrual Period (LMP) — not from the date of conception. This standardized dating method is used by all healthcare providers and medical guidelines worldwide. Because ovulation and fertilization typically occur around day 14 of a 28-day cycle, gestational age is approximately 2 weeks ahead of actual fetal age (from fertilization). A full-term pregnancy is defined as 39–40 weeks gestational age. Babies born at 37–38 weeks are considered early term, 39–40 weeks are full term, 41 weeks are late term, and 42+ weeks are post-term. This calculator tells you your current gestational week and trimester based on your LMP date.
How to Use This Pregnancy Week Calculator
- Step 1 — Enter your Last Menstrual Period (LMP) date: This is the first day of your most recent period before becoming pregnant. If you don't know the exact date, use the closest estimate.
- Step 2 — Click Calculate to instantly see your current gestational week, trimester, estimated due date, and key developmental milestones for your baby.
If your ultrasound gestational age differs from your LMP-based calculation by more than 1–2 weeks, your healthcare provider will typically adjust your due date to match the ultrasound measurement (especially when done before 13 weeks). Early ultrasound dating is more accurate than LMP-based dating in many women.
Your Pregnancy Week by Week — The Three Trimesters
Pregnancy is divided into three trimesters, each with distinct developmental and physical milestones:
- First Trimester (Weeks 1–13): The most critical period for fetal organ formation (organogenesis). The heart, brain, spinal cord, and limbs develop. Most miscarriages (80%) occur in this trimester. Morning sickness, fatigue, and breast tenderness are common. Prenatal vitamins with folic acid are essential for neural tube development.
- Second Trimester (Weeks 14–26): Generally the most comfortable trimester. Morning sickness typically improves. The baby's movements (quickening) are felt from weeks 16–20. Anatomy ultrasound (18–22 weeks) checks for structural abnormalities. Gender can usually be determined. Baby develops hearing and can respond to sounds.
- Third Trimester (Weeks 27–40): Rapid weight gain for both mother and baby. Baby's lungs mature and brain develops rapidly. Braxton Hicks contractions may begin. Baby typically moves into a head-down position by week 36. Birth preparedness classes, hospital tours, and birth plan discussions should occur in this trimester.
Essential Prenatal Health Checklist
- Folic Acid: 400–800 mcg daily before conception and through the first trimester reduces neural tube defects (like spina bifida) by up to 70%.
- Prenatal Vitamins: Include iron (27 mg), calcium (1,000 mg), vitamin D (600 IU), and DHA (200 mg) for fetal brain and eye development.
- Avoid: Alcohol (no safe amount during pregnancy), smoking (increases risk of miscarriage, preterm birth, low birthweight), raw or undercooked meat, unpasteurized cheese, high-mercury fish, and deli meats.
- Exercise: 150 minutes of moderate activity per week is safe and beneficial. Walking, prenatal yoga, and swimming are excellent. Avoid contact sports and exercises with fall risk after the first trimester.
- Prenatal Appointments: Monthly visits from conception to 28 weeks; every 2 weeks from 28–36 weeks; weekly from 36 weeks to delivery. Do not skip glucose screening (24–28 weeks) for gestational diabetes.
Frequently Asked Questions
Why is pregnancy counted from the last period?
Gestational age is counted from the LMP because the exact date of conception is usually unknown, but the LMP is reliably recalled. This means you are technically "pregnant" for 2 weeks before conception actually occurs. Ultrasound measurements in early pregnancy may adjust the due date if they differ from LMP calculations.
How accurate is my due date?
Only about 5% of babies are born on their exact due date. Most arrive within a 2-week window on either side. A due date is an estimate; births from 37–42 weeks are considered normal. Your healthcare provider will monitor progress and advise on induction if pregnancy extends significantly past the due date.
When should I have my first prenatal appointment?
Most guidelines recommend the first prenatal visit between 8–10 weeks. It typically includes a confirmation of pregnancy, blood tests, blood pressure check, urine test, and discussion of prenatal vitamins. If you have risk factors, your provider may want to see you earlier.