How Gestational Age Is Calculated
Pregnancy is measured in gestational weeks starting from the first day of your last menstrual period (LMP) — not from the date of conception. This convention dates to before modern understanding of ovulation, when LMP was the only reliably known date. Because ovulation typically occurs about 14 days after LMP, gestational age is approximately 2 weeks more than the embryo's developmental age. When a doctor says you're 8 weeks pregnant, the embryo is developmentally about 6 weeks old.
The standard gestational period is 40 weeks (280 days) from LMP, calculated using Naegele's rule: add 1 year, subtract 3 months, add 7 days to the LMP date. For cycle lengths other than 28 days, adjust by the difference: a 30-day cycle shifts the due date forward by 2 days. Full term is defined as 39 weeks 0 days through 40 weeks 6 days; preterm is before 37 weeks; post-term is 42 weeks or later.
Early ultrasound remains the gold standard for dating accuracy. A first-trimester ultrasound measuring crown-rump length (CRL) is accurate to ±3–5 days — more precise than LMP dating, which depends on accurate recall and assumes regular cycles. If the ultrasound estimate differs from LMP-based dating by more than 5–7 days in the first trimester (or more than 2 weeks after 14 weeks), the ultrasound date typically takes precedence.
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Calculate Due Date & MilestonesWhat Happens Each Trimester
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First Trimester (Weeks 1–13): Formation
All major organ systems form during the first trimester. The heart begins beating by week 6. By week 10, the embryo is called a fetus and all essential structures are present. Miscarriage risk is highest in this trimester (approximately 10–20% of known pregnancies), dropping significantly after week 12. Symptoms typically include fatigue, nausea, breast tenderness, and frequent urination. Key appointments: confirmation ultrasound, first prenatal labs, nuchal translucency scan (weeks 11–14).
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Second Trimester (Weeks 14–27): Growth
The second trimester is often the most comfortable — nausea typically resolves, energy returns, and the pregnancy becomes visible. The fetus grows rapidly, and most women feel movement (quickening) between weeks 16–22. The anatomy scan at week 18–22 is the most comprehensive structural assessment. Key tests: quad screen (week 16), anatomy ultrasound (week 20), glucose challenge test (week 24–28).
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Third Trimester (Weeks 28–40+): Preparation
The third trimester focuses on fetal growth, lung maturation, and positioning for birth. The baby gains approximately half a pound per week. Braxton Hicks contractions become more noticeable. Key appointments become weekly after week 36. Key tests: Group B Strep swab (week 35–37), biophysical profile if needed, non-stress tests if post-dates. Birth plan discussions and hospital pre-registration happen in this trimester.
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Post-dates (After 40 Weeks): Monitoring
Pregnancies extending beyond 40 weeks are monitored more closely. Non-stress tests and biophysical profiles assess fetal wellbeing. Most providers recommend induction by 41–42 weeks due to increasing risk of stillbirth and other complications. The placenta functions less efficiently after 40 weeks, and amniotic fluid tends to decrease. Discuss your provider's induction policy before you reach your due date.
Frequently Asked Questions
What is the anatomy scan and what does it check?
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The anatomy scan (mid-pregnancy ultrasound) at 18–22 weeks checks fetal anatomy systematically: brain structure, face, spine, heart (four chambers, outflow tracts), abdominal organs, kidneys, bladder, limbs, and cord insertion. It also assesses amniotic fluid volume, placental location (checking for previa), and confirms gestational age. Sex determination is possible if the baby cooperates. The scan takes 30–60 minutes and is performed by a sonographer, with results reviewed by a radiologist or your OB.
What is NIPT and when can it be done?
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Non-invasive prenatal testing (NIPT) analyzes cell-free fetal DNA circulating in maternal blood to screen for chromosomal conditions including trisomies 21 (Down syndrome), 18, and 13, as well as sex chromosome abnormalities. It can be performed from 10 weeks onward. NIPT is a screening test, not diagnostic — positive results require confirmation with diagnostic testing (CVS or amniocentesis). NIPT can also determine fetal sex from the presence or absence of Y chromosome sequences.
When should I call my doctor about symptoms?
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Contact your provider immediately for: heavy vaginal bleeding, severe abdominal pain, sudden severe headache with visual changes or facial swelling (preeclampsia signs), significant decrease in fetal movement after 28 weeks, fluid gushing or trickling from vagina (possible rupture of membranes), signs of preterm labor before 37 weeks (regular contractions, pelvic pressure, low backache). In the third trimester, if you're unsure whether you're in labor, always call — providers would rather hear from you than not.
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