Medical disclaimer: This article is general educational information. Your individual prenatal schedule depends on health history, risk factors, and provider preference — always consult your obstetrician or midwife. This tool does not replace medical advice.
A first positive home pregnancy test starts a 40-week loop of the same questions every week: “How big is the baby this week?”, “When is my next appointment?”, “Should I do cell-free DNA?”, “When does the anatomy scan happen?”, “When do I tell HR?”. A printed appointment card from the OB office shows the next visit but never the whole 40-week picture. One tool that takes a single starting point — your last menstrual period, ovulation date, or IVF transfer date — and lays out every prenatal visit on one screen is what fills that gap.
How due dates are calculated — Naegele’s rule and ultrasound
The most common method in US obstetrics is Naegele’s rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). The rule assumes a textbook 28-day cycle with day-14 ovulation, so a ±2-week range is normal.
ACOG considers a first-trimester ultrasound (8-13 weeks) measuring crown-rump length (CRL) to be the most accurate dating method — under 5 days of error. If your LMP estimate and ultrasound disagree by more than 7 days in the first trimester, the ultrasound dating becomes the official due date.
If you have irregular cycles, were breastfeeding, or recently stopped birth control, use a different starting point:
| Starting point | Formula | Best for |
|---|---|---|
| LMP (last period) | + 280 days | Regular 28-day cycle |
| Ovulation date | + 266 days | Irregular cycle, unknown LMP |
| IVF Day-3 transfer | + 263 days | Cleavage stage IVF |
| IVF Day-5 blastocyst | + 261 days | Blastocyst transfer IVF |
The pregnancy due date calculator supports all four. All math runs in your browser — your LMP and IVF dates never leave the device.
First prenatal visit through delivery — the ACOG cadence
ACOG’s standard prenatal visit cadence is monthly through 28 weeks, biweekly from 28 to 36 weeks, and weekly after 36 weeks — about 14 visits total for a low-risk pregnancy. Specific tests slot in at specific weeks.
| Week | Visit / Test | Notes |
|---|---|---|
| 8-10 | First prenatal visit | Dating ultrasound, bloodwork (CBC, blood type, infectious disease, immunity) |
| 10-13 | First trimester screen | NT ultrasound + maternal serum (PAPP-A, hCG) |
| 10+ | Cell-free DNA (cfDNA) | ACOG offers to all pregnant patients |
| 11-14 | Chorionic villus sampling (CVS) | Optional diagnostic if cfDNA positive |
| 15-20 | Quad screen (alternative to cfDNA) | If cfDNA not done |
| 18-22 | Anatomy scan (Level II ultrasound) | 30-60 minute structural check |
| 24-28 | Glucose challenge test (GCT 50g) | Two-step approach standard |
| 28 | Switch to biweekly visits | Rh- patients get RhoGAM |
| 28-36 | Tdap vaccine | Whooping cough antibodies for newborn |
| 36-37 | Group B strep (GBS) screening | Vaginal-rectal swab |
| 36 | Switch to weekly visits | Position check, cervix exam |
| 39-41 | Delivery window | 40 ±2 weeks normal |
Screenshot the result page and text it to your partner — that’s the most common workflow first-time mothers describe. The schedule never has to be retyped.
Cell-free DNA (cfDNA) and the first trimester screen
These two tests are commonly confused but are different.
Cell-free DNA (cfDNA / NIPT)
- Timing: 10 weeks onward
- Test: Single maternal blood draw
- Detects: Trisomy 21, 18, 13 (and optionally sex chromosome aneuploidies)
- Sensitivity: 99%+ for Down syndrome
- Cost: $200-$2,000 out-of-pocket if not covered (insurance coverage varies)
- ACOG 2020 update: now offered to all pregnant patients regardless of age
First trimester screen
- Timing: 10-13 weeks
- Test: Nuchal translucency (NT) ultrasound + maternal blood markers (PAPP-A, free β-hCG)
- Output: Risk score (e.g., 1 in 250)
- Sensitivity: 80-85% for Down syndrome
- Cost: typically covered by insurance under prenatal care
Both are screening tests, not diagnostic. A positive result must be confirmed with amniocentesis (15-20 weeks, 1 in 500 miscarriage risk) or CVS (11-14 weeks). Many parents do cfDNA at 10 weeks and skip the first trimester screen entirely.
The anatomy scan — what gets checked at 18-22 weeks
The Level II anatomy ultrasound is the most thorough scan of the pregnancy. Over 30-60 minutes, the sonographer documents:
- Head and brain: skull shape, ventricles, cerebellum, posterior fossa
- Face: lips and palate (cleft check), profile, eye sockets
- Heart: four chambers, outflow tracts, three-vessel view
- Spine: full length for neural tube defects
- Abdomen: stomach, kidneys, bladder, abdominal wall (gastroschisis check)
- Limbs: long bones measured, hands and feet counted
- Placenta and amniotic fluid: position, AFI
About half of major structural anomalies are first identified at this scan. If something flags, your provider may order a fetal echocardiogram (24-28 weeks) or refer to maternal-fetal medicine.
Most insurance plans cover this scan as standard prenatal care. Many parents add an optional 3D/4D ultrasound at 26-30 weeks for keepsake images — typically $100-$300 out-of-pocket at non-medical centers.
Glucose tolerance — the two-step approach
Most US practices use the two-step approach at 24-28 weeks.
Step 1 — 50g glucose challenge test (GCT)
- No fasting required (some practices recommend not eating sugary breakfast)
- Drink 50g glucose solution
- Wait 1 hour
- Single blood draw
- Cutoff varies: 130, 135, or 140 mg/dL depending on practice
- About 15-25% fail step 1
Step 2 — 100g glucose tolerance test (GTT)
- 8-hour fast required
- Fasting blood draw, then 50g + 100g glucose
- Blood draws at 1, 2, and 3 hours
- Two abnormal values out of four = gestational diabetes diagnosis
About 7% of US pregnancies are diagnosed with gestational diabetes. Treatment starts with diet (carbohydrate counting, smaller frequent meals) and home glucose monitoring. About 15-20% of women with gestational diabetes need insulin (oral medications like metformin are sometimes used but insulin remains the standard). Risk factors that can prompt earlier screening at the first prenatal visit: BMI >30, prior gestational diabetes, family history, prior macrosomic baby (>9 lb), or PCOS.
GBS screening at 36-37 weeks — why so late
The vaginal-rectal swab for Group B strep (GBS) happens at 36-37 weeks. About 25% of US women are GBS-positive. If positive, you receive IV penicillin during labor, which reduces newborn early-onset GBS infection (sepsis, meningitis) by over 99%.
The reason GBS is screened late: bacterial colonization can come and go. A test at 28 weeks might not reflect status at delivery. The 36-37 week window catches colonization close enough to delivery to be predictive while still leaving time for results before labor.
FMLA, paid leave, and the HR conversation
US maternity leave is a patchwork — there is no federal paid leave.
FMLA (Family and Medical Leave Act)
- 12 weeks unpaid, job-protected leave per 12-month period
- Eligibility: 1,250 hours worked over past 12 months at employer with 50+ employees within 75 miles
- Health insurance maintained at the same cost
- Same or equivalent position when you return
State paid family leave (varies)
- California, New York, New Jersey, Rhode Island, Washington, Massachusetts, Connecticut, Oregon, Colorado offer paid leave (typically 8-12 weeks at partial pay)
- Other states: nothing at the state level
Short-term disability (STD)
- Often through employer or private policy
- Typically covers 6 weeks vaginal birth / 8 weeks C-section
- Pays 50-80% of salary
The HR conversation: notify HR around 20 weeks so they can plan coverage, but you can disclose earlier if you need accommodations. Get FMLA paperwork from HR; your OB completes the certification with your due date and projected leave start. Combine FMLA + STD + state paid leave + saved PTO to maximize total leave time. If you save the result page from the pregnancy due date calculator showing your due date and projected leave start, the HR conversation is a 5-minute meeting instead of a 30-minute back-and-forth.
The weekly fetal development card — the screen you screenshot
The other half of the tool is the weekly fetal development card. Based on your starting point, the current week’s fetal size (cm and grams) and key milestones display. Example: 20 weeks ≈ 25 cm / 300 g, sex visible on ultrasound, hair starts growing.
Most users screenshot this card and text it to their partner weekly. The card doesn’t need rewording — fits in a single message without character count tools needed for trimming.
URL sharing also works. The format /en/pregnancy_calc/?start=lmp&date=2026-01-15 reproduces the same result so your partner sees the identical screen.