Pregnancy week calculator — what your numbers actually mean

This tool calculates your current pregnancy week and estimated due date from one of three starting points: last menstrual period (LMP), ovulation date, or IVF transfer date. All math runs in your browser — your dates are never sent to our servers.

Naegele's rule — the 280-day formula

OB/GYNs in the US (and worldwide) use Naegele's rule: due date = LMP + 280 days (40 weeks). It assumes a 28-day cycle. If your cycles run 30-35 days, your actual due date will be a few days later. The first-trimester ultrasound (8-12 weeks) measures crown-rump length (CRL) and adjusts the due date within ±5 days of the calculated value — that ultrasound date is what your provider will use going forward.

ACOG / CDC prenatal schedule

• 6-10 wk: First prenatal visit. Confirm pregnancy, dating ultrasound, prenatal vitamin.
• 10-13 wk: NIPT (cell-free DNA) or CVS — chromosomal screening (Down, Edwards, Patau).
• 11-14 wk: Nuchal translucency scan — first-trimester combined screen.
• 15-20 wk: Quad screen (optional, less common since NIPT availability).
• 18-22 wk: Anatomy ultrasound ("20-week scan") — fetal anatomy + sex visible.
• 24-28 wk: 1-hour glucose tolerance test (GTT) for gestational diabetes screen.
• 35-37 wk: Group B strep (GBS) culture.
• 39-41 wk: Estimated delivery window. Induction usually offered at 41 weeks.

Trimester-by-trimester essentials

• First trimester (~12 wk): Folic acid 400-800 mcg/day (prevents neural tube defects). Limit caffeine to <200 mg (~12 oz brewed coffee). Skip alcohol, raw fish, deli meats, soft cheeses. Nausea peaks at 6-12 weeks for most.
• Second trimester (13-27 wk): The "honeymoon phase." Energy returns, nausea fades, you'll feel first movements (quickening) around 18-22 weeks. Schedule the 20-week anatomy scan and the GTT.
• Third trimester (28+ wk): Rapid growth, back pain common. Pack hospital bag by 36 weeks. Learn labor signs: water break, regular contractions <5 minutes apart for an hour.

NIPT vs quad screen — what, when, and why

Two main blood-based screens look at fetal chromosomes: NIPT (cell-free DNA) analyzes fetal DNA fragments in maternal blood for Down, Edwards, and Patau syndromes, and the quad screen measures AFP, hCG, uE3, and inhibin A. Both are screening tests — a positive result requires confirmatory CVS (11–13 wk) or amniocentesis (16+ wk).

• NIPT: from 10 wk / 99%+ accuracy for Down / often covered by insurance for high-risk patients, $0–800 out of pocket
• Quad screen: 15–20 wk / ~80% accuracy / $50–200 / less common since NIPT availability
• NT scan + first-trimester combined: 11–14 wk / ~85% accuracy / $100–300

ACOG recommends offering aneuploidy screening to all pregnant patients regardless of age. NIPT is preferred for patients ≥35 years, prior chromosomal abnormality, or positive first-trimester screen.

The 20-week anatomy scan — 30 to 60 minutes of detail

The anatomy ultrasound at 18–22 weeks is the longest scan of your pregnancy. The sonographer methodically images brain, heart, spine, kidneys, abdomen, limbs, and genitalia for 30–60 minutes to screen for structural anomalies. Common findings:

• Cleft lip / palate — detailed facial views
• Ventricular septal defect (VSD) — 4-chamber view + outflow tracts
• Hydronephrosis — renal pelvis measurement
• Spina bifida — spinal alignment + cranial signs
• Omphalocele / diaphragmatic hernia — abdominal wall continuity

Cost: $200–500 with insurance. This is also when most providers can confirm fetal sex if you want to know.

Gestational diabetes — the 1-hour GCT and the 3-hour GTT

Most US OB practices use a 2-step approach at 24–28 weeks:

• Step 1 — 1-hour 50g GCT: no fasting, drink 50g glucose, blood draw 1 hr later. Cutoff usually 140 mg/dL.
• Step 2 — 3-hour 100g GTT: required only if Step 1 is elevated. 8-hour fast + fasting draw + 100g load + 1, 2, 3 hr draws. Diagnosis: 2+ values above threshold.

US gestational diabetes rate is 6–9%. First-line treatment is medical nutrition therapy (carb distribution) + self glucose monitoring. Insulin or metformin if diet alone doesn't achieve targets. Uncontrolled GDM raises risk of macrosomia, neonatal hypoglycemia, and maternal type 2 diabetes later in life.

Group B strep (GBS) — 35–37 weeks, why it matters

GBS culture at 35–37 weeks is a simple vaginal/rectal swab. About 10–30% of pregnant patients carry GBS, which is harmless for the mother but can cause neonatal sepsis or meningitis if transmitted during vaginal birth. Standard CDC protocol: IV penicillin every 4 hours from the start of labor (IAP) reduces neonatal early-onset GBS disease by approximately 80% (CDC). Don't push the test past 37 weeks since labor (or PROM) can come first.

The full US prenatal visit schedule

ACOG-standard frequency for a low-risk pregnancy:

• Through 28 wk: every 4 weeks (~7 visits)
• 28–36 wk: every 2 weeks (~4 visits)
• 36 wk to delivery: every week (~4–5 visits)

That's roughly 14 prenatal visits total, with the specific screening tests above layered in. Our tool's "Full 40-week timeline" section maps all of them on a single screen.

IVF pregnancies — Day 3 vs Day 5 transfer

For IVF pregnancies, the transfer day matters: a Day 3 (cleavage stage) embryo is converted to LMP equivalent by subtracting 17 days; a Day 5 (blastocyst) embryo by subtracting 19 days. Day 5 blastocyst transfer is the modern standard at most US fertility clinics. This tool supports both options.

Insurance, FMLA, and the practical timeline

Confirm with your insurance: pregnancy is a "qualifying life event," so you may want to update coverage within 30 days of confirmation. Eligible employees in the US get up to 12 weeks unpaid leave under FMLA. State paid family leave varies — California, New York, New Jersey, Massachusetts, Washington, Colorado, Connecticut, Oregon, and Rhode Island offer some paid leave. Check your state's program.

Why the due date is just an estimate

Only about 4% of babies arrive exactly on the due date. Most full-term babies arrive between 37 and 42 weeks, with the median at 40 weeks 5 days for first-time mothers per ACOG. Your provider will discuss induction options if you go past 41 weeks.

Three pregnancy-week traps that confuse first-time parents

1. LMP vs IVF vs ultrasound — three reference points, three possible due dates

American obstetric standard practice uses LMP (Last Menstrual Period) as the first dating method via Naegele's rule (LMP + 280 days). IVF dating is more precise — embryo transfer date + 14-day correction (for blastocyst transfer). First-trimester ultrasound (CRL measurement, weeks 7–13) is the gold standard, and ACOG (Practice Bulletin #146) recommends adjusting EDD to ultrasound if it differs from LMP by more than 7 days. This tool accepts all three inputs because the right one varies by patient — irregular cycles (not 28 days) reduce LMP accuracy, making ultrasound primary.

2. EDD ± 2 weeks is normal — only ~4% deliver on the exact predicted date

ACOG defines full-term as 37 weeks 0 days through 41 weeks 6 days, with EDD as the midpoint. Roughly 80% of singleton births occur between weeks 38 and 40+6, and only about 4% actually deliver on the EDD itself. First-time mothers tend to go past EDD — the median for first births is about EDD + 8 days (Jukic 2013, Human Reproduction). "I'm overdue" rarely means actually overdue until you cross 42+0 weeks (post-term), which is when active induction is typically discussed. This tool's EDD is a population average; first vs subsequent births, parity, and cervical readiness all shift the actual date by ±1–2 weeks.

3. "9 months" vs "40 weeks" — the math doesn't match what you'd expect

People say "9 months pregnant" but obstetric weeks total 40, which is closer to 9 months 7 days on the calendar — not exactly 9 calendar months. The difference: a "lunar month" in obstetric usage is 28 days, so 10 lunar months = 280 days = 40 weeks. Calendar months average 30.4 days, so 40 weeks = 280 / 30.4 ≈ 9.2 calendar months. All clinical decisions (NT scan at 11–13 weeks, anatomy scan at 18–22, GBS at 36) are scheduled by weeks, not months. This tool shows weeks + days + lunar months together so you have the right unit for each context.

⚠️ Educational use only. This tool does not replace medical advice. Confirm dating, schedule, and any symptoms with your healthcare provider. Sources: ACOG Practice Bulletin #145 / CDC / Mayo Clinic / NICHD.