Pregnancy Week-by-Week Calculator — Gestational Age + Milestones

What This Calculator Does

The Pregnancy Week-by-Week Calculator returns your current gestational age (weeks + days) and estimated due date (EDD)from any of four common starting points — first day of your last menstrual period (LMP), conception date, a known due date your provider has already given you, or a first-trimester ultrasound’s crown-rump-length (CRL) measurement. It also surfaces fetal age, the conception date estimate, the 24-week viability milestone, and the full-term window (37–42 weeks).

The companion week-by-week milestone reference panel below the result lets you scrub forward through any gestational week (4–42) to see what’s typical at that point — fetal size comparison, approximate length and weight, common physical or emotional experiences, and the standard clinical appointments / tests scheduled for that week. The milestone content is non-medical and conservative: descriptive, not diagnostic; references typical patient-education timelines (ACOG, Mayo Clinic, NHS Start4Life), not your specific pregnancy.

The Four Calculation Methods

1. LMP — first day of last menstrual period (most common)

Naegele’s rule has been the obstetric standard since 1830. It assumes a 28-day cycle with ovulation on day 14, so a fertilized egg already has ~2 weeks of gestational age the moment it implants. Every clinician, ultrasound report, and prenatal record uses LMP-based dating. Accuracy: ±5-7 daysfor women with regular cycles, less for irregular ones. If your cycle isn’t 28 days, the simpler Pregnancy Due Date Calculator accepts cycle-length adjustment; this calculator uses the textbook 28-day assumption.

2. Conception date — IVF transfer or charted cycle

266 days = 280 days from LMP minus the 14 days between LMP and ovulation. Use this method when you know the exact conception date — IVF embryo transfer dates, or carefully charted ovulation cycles. For Day-3 IVF transfers, no adjustment is typically needed; for Day-5 (blastocyst) transfers, some clinics shift the LMP-equivalent by 2-3 days. Your fertility clinic’s date is what your medical record will use, and this calculator matches the same convention.

3. Known due date — reverse-calculate

If your provider has already given you a confirmed EDD (often from an early-trimester ultrasound), the simplest way to track your current week is to plug that EDD in. The calculator reverse-calculates the LMP-equivalent date and then computes today’s gestational age from it. This is the most accurate path when ultrasound dating has already corrected for irregular cycles or LMP uncertainty.

4. Ultrasound CRL — Robinson-Fleming (most accurate)

Crown-rump length (CRL) is the straight-line measurement from the top of the embryo’s head to its bottom, taken on a first-trimester ultrasound. Robinson and Fleming’s 1975 regression — validated on 334 pregnancies and the ACOG-recommended dating standard ever since — converts CRL directly to gestational age in days, accurate to ±3-5 days within the 6-84 mm range (≈ 6½ to 14 weeks).

Outside that CRL range, ultrasound dating switches to second-trimester biometric measurements (biparietal diameter, head circumference, femur length) which the calculator doesn’t accept. If your scan is past 14 weeks, use the LMP or known-due-date method instead.

A Worked Example — “LMP 2026-01-20”

Say your LMP was January 20, 2026. Today (April 27, 2026):

• Days since LMP: 97 days
• Gestational age today: 13 weeks 6 days — start of second trimester
• Estimated due date: January 20 + 280 days = October 27, 2026
• Days to EDD: 183 days · 26 weeks remaining
• Fetal age (post-conception): 11w 6d
• Estimated conception date: February 3, 2026 (LMP + 14 days)
• Viability milestone (24w0d): July 7, 2026
• Full-term window (37-42w): October 6 – November 10, 2026

At this point in the pregnancy, the upcoming clinical milestones are the second-trimester maternal serum screen (15-20w), the anatomy scan (18-22w), and the glucose challenge (24-28w). The week-by-week milestone panel below the calculator surfaces all of these — scrub forward to your current week or any future appointment week.

Trimester Boundaries

• 1st trimester:0w 0d – 12w 6d (some sources extend to 13w 6d). Highest miscarriage risk; major organ development; nausea / fatigue peak for many.
• 2nd trimester:13w 0d – 26w 6d. The “easier” window for many pregnancies — energy returns, nausea fades, movement begins to be felt around 16-22w. Anatomy scan and most major screening tests happen here.
• 3rd trimester: 27w 0d onward. Within it: early term 37w 0d – 38w 6d, full term39w 0d – 40w 6d, late term 41w 0d – 41w 6d, post-term 42w 0d+. Most providers begin discussing induction at 41 weeks and induce by 42 weeks.

The Viability Milestone — 24 Weeks

24w 0d is widely recognized as the threshold of extreme-preterm viability. Most NICUs in developed countries can support a baby born at 24 weeks, with survival rates that improve dramatically week by week:

• 23w 0d: ~30-50% survival rate (varies widely by NICU and individual circumstances)
• 24w 0d:~60-70% survival rate — the formal threshold for “viable” in most clinical guidelines
• 25w 0d: ~70-80% survival rate
• 28w 0d: ~95% survival rate
• 32w 0d: ~99% survival rate

These are aggregate survival figures; long-term outcomes (developmental, neurological) vary independently of survival. The calculator surfaces the 24-week date as a planning anchor. It is not a guarantee or a clinical prediction — individual outcomes depend on dozens of factors only your medical team can assess.

Why Different Apps Show Different Weeks

It’s very common for pregnancy apps to disagree by a day or two. Three usual causes:

1. LMP-based vs ultrasound-based dating.Your provider may have switched to ultrasound dating after the first scan if it disagreed with LMP by more than 7 days (ACOG’s recommended threshold). Apps anchored to LMP will continue showing the older number.
2. Cycle-length assumptions. This calculator uses 28 days for LMP. Some apps assume your charted average (e.g. 30 or 32 days) and adjust the EDD accordingly.
3. Time-zone boundaries. The exact moment you cross from 14w 6d to 15w 0d is calendar-day-aligned; two apps anchored to slightly different time zones can show different weeks for a few hours around midnight.

Differences of 1-3 days are normal and don’t affect care. Differences over 5 days mean the apps are using different reference dates — go with whichever your provider has marked in your medical record.

Common Mistakes

Confusing fetal age with gestational age

Gestational age is from LMP. Fetal age is from conception. They’re different by ~14 days. Every clinical record uses gestational age — if a pregnancy app says “your baby is 12 weeks” from conception, you’re actually 14 weeks gestational. The calculator uses gestational age throughout (matching medical convention) and surfaces fetal age as a separate detail line for completeness.

Picking the wrong “reference date”

The reference-date field changes meaning based on the method: for LMP it’s the first day of bleeding; for conception it’s the day fertilization happened (or IVF transfer); for known due date it’s your provider’s EDD; for CRL it’s the date of the ultrasound scan. Mixing them up produces a wildly wrong gestational age.

Using CRL outside the valid range

Robinson-Fleming is calibrated for CRL between 6 mm and 84 mm (≈ 6½ to 14 weeks). Outside that range the formula extrapolates poorly. The calculator rejects out-of-range values — switch to LMP or known-due-date for second-trimester ultrasounds.

Treating the EDD as a deadline

About 50% of births happen on or after the calculated EDD. Only 4-5% happen exactly on it. The EDD is the midpoint of a natural distribution centered around 40w 0d, with 80% of births in the 38-42 week window. The calculator’s “X days to EDD” line is a planning anchor, not a countdown timer.

What This Calculator Doesn’t Do

• It doesn’t adjust for cycle length on the LMP method.If your cycle isn’t 28 days, use the simpler Pregnancy Due Date Calculator which accepts a cycle-length input.
• It doesn’t handle multiples differently. Twin pregnancies typically deliver 2-4 weeks before singletons; triplets earlier still. The math returns the singleton-equivalent EDD; your specialist sets the practical timeline.
• It doesn’t make any clinical recommendation. The milestone content is descriptive (size, length, weight, appointments). No diet, medication, supplement, or exercise advice is offered. For all of that, your provider is the source.
• It doesn’t replace your prenatal record. When LMP and ultrasound dating disagree, your provider’s chart is what your care plan uses. The calculator helps you understand the math; the chart drives the medicine.

Related Calculators

• Pregnancy Due Date — simpler EDD-only calc with cycle-length adjustment for non-28-day cycles. Use it if you don’t need the week-by-week milestone reference.
• Ovulation — fertility window timing for trying to conceive, including irregular-cycle support.
• TDEE — baseline maintenance calorie estimate. Pregnancy guidance varies by trimester (~340 cal/day extra in T2, ~450 cal/day in T3 per ACOG general guidance) — talk to your provider for your specific situation.
• Protein Intake — daily protein target by bodyweight + activity. Pregnancy increases protein needs (~+25 g/day in T2 and T3 per standard guidance) — confirm with your provider.

How to Read the Result

Three numbers anchor every appointment: gestational age (weeks + days), EDD (estimated due date), and trimester boundary (which appointments and screenings come up next). The dating method matters — first- trimester ultrasound CRL is the most accurate, then LMP, then conception-date if certain.

• LMP-derived EDD differs from ultrasound EDD by ≥7 days (first trimester). Your provider will redate the pregnancy using ultrasound — CRL is the gold standard early. Update calendar reminders accordingly.
• Currently before week 24. Pre-viability window. Flag any decision-relevant appointments (NIPT 10-13 weeks, anatomy scan 18-22 weeks) on the calendar — these are the windows that close.
• Currently 37-42 weeks.Full-term window. Don’t panic at 41+0 — only 5% of births land on the EDD; 80% deliver between 38-42 weeks. Keep the bag packed, stay in routine contact with your provider.
• Cycles irregular OR LMP unknown.Wait for first ultrasound — the calculator’s LMP method assumes a regular 28-day cycle and 14-day ovulation, which can be off by weeks for irregular cycles.