Pregnancy Calculator
Enter your last period date, due date, conception date, ultrasound data, or IVF transfer date to calculate your estimated due date, current pregnancy week, trimester, and every key prenatal screening appointment window.
Enter your values above to see the results.
Tips & Notes
- ✓The due date is the midpoint of a normal delivery window — only 5% of babies arrive on the exact date. Weeks 37–42 are all considered full term, so focus on the window, not the day.
- ✓First-trimester ultrasound (ideally 8–13 weeks) provides the most accurate gestational age dating — more accurate than LMP calculation, especially if you have irregular cycles.
- ✓If your LMP-based due date and ultrasound-based due date differ by more than 7 days (in the first trimester), your obstetrician will typically update the due date to the ultrasound measurement.
- ✓Track which prenatal screenings are coming up by week — the NT scan window (weeks 11–14) and NIPT window close relatively early, and missing them means waiting for second-trimester screening options.
- ✓Gestational age is always counted from LMP, not from conception or the positive pregnancy test. When your provider says "you are 8 weeks pregnant," they mean 8 weeks from LMP, which is approximately 6 weeks from conception.
Common Mistakes
- ✗Counting pregnancy weeks from the positive pregnancy test — by the time most people get a positive test, they are already 4–6 weeks pregnant (by gestational age from LMP).
- ✗Assuming the due date is exact — planning around a single date ignores the biological reality that normal delivery spans weeks 37–42, and any date in that range is a healthy outcome.
- ✗Using LMP-based dating when cycles are irregular — for cycles that vary significantly in length or are consistently much shorter or longer than 28 days, ultrasound dating is more reliable.
- ✗Confusing gestational age with fetal age — providers use gestational age (from LMP) which is 2 weeks longer than fetal age (from conception). When you say "10 weeks pregnant," the embryo is actually about 8 weeks old.
- ✗Missing the nuchal translucency scan window — this first-trimester screen (weeks 11–14) has a hard cutoff and cannot be performed later. After week 14, different screening options apply.
Pregnancy Calculator Overview
The due date is the starting point, but pregnancy is really a 40-week calendar of milestones, screenings, and development events. Knowing your week tells you exactly what is happening and what comes next.
Gestational age and due date calculation:
Naegele's Rule (standard due date calculation): Due Date = First day of LMP + 280 days (40 weeks) Adjustment for non-28-day cycles: Due Date shifts by (actual cycle length − 28) days Conception date estimate = LMP + 14 days (assumes ovulation at day 14)
EX: LMP = March 15, cycle length = 30 days (2 days longer than average) Standard Naegele: March 15 + 280 days = December 20 Cycle adjustment: +2 days → Adjusted Due Date = December 22 Current gestational age (if today = July 10): 17 weeks 1 day Trimester: Second (weeks 13–26) Anatomy scan due: approximately week 18–20 → late July to early August
Trimester boundaries and fetal development:
Trimester boundaries (gestational weeks from LMP): First trimester: Weeks 1–12 (organ formation, highest miscarriage risk) Second trimester: Weeks 13–26 (fetal movement begins, anatomy scan) Third trimester: Weeks 27–40 (rapid growth, lung maturation, birth preparation) Full term: Week 37–42 (delivery within this window is considered full term)
EX: IVF frozen embryo transfer, Day 5 blastocyst transferred on April 10 Gestational age at transfer: 2 weeks 5 days (Day 5 embryo = 5 days post-fertilization = ~19 days post-LMP equivalent) LMP equivalent = April 10 − 19 days = March 22 Due Date = March 22 + 280 days = December 27 Current week at 10 weeks after transfer: approximately Week 17 of pregnancy
Key prenatal milestones and appointment timeline:
| Gestational week | Prenatal milestone | What happens |
|---|---|---|
| Week 6–8 | First prenatal visit | Dating ultrasound, blood tests, Rh factor, full history |
| Week 11–14 | Nuchal translucency (NT) scan | First-trimester Down syndrome and chromosomal screening |
| Week 11–13 | NIPT (optional) | Non-invasive prenatal testing — chromosomal analysis from maternal blood |
| Week 15–20 | Quad screen (optional) | Second-trimester blood test for chromosomal and neural tube defects |
| Week 18–22 | Anatomy scan (20-week scan) | Detailed fetal structure assessment, sex can be determined |
| Week 24–28 | Glucose tolerance test (GDM screen) | Gestational diabetes screening — 1-hour or 2-hour glucose challenge |
| Week 28 | Rh immunoglobulin (if Rh negative) | Rhogam injection to prevent Rh sensitization |
| Week 35–37 | Group B Strep (GBS) swab | Vaginal/rectal swab to determine need for antibiotics in labor |
| Week 37 | Full term milestone | Baby considered full term — delivery no longer preterm |
| Week 41–42 | Post-dates monitoring | Non-stress tests, biophysical profile, induction discussion |
Fetal development by trimester:
| Gestational week | Fetal size | Weight | Key development |
|---|---|---|---|
| Week 8 | 2.5 cm | 3 g | All major organs forming; fingers beginning |
| Week 12 | 6 cm | 14 g | Fingers and toes formed; heartbeat audible on doppler |
| Week 20 | 25 cm | 300 g | Hearing develops; can respond to sound |
| Week 24 | 30 cm | 600 g | Viability threshold — survival possible with intensive NICU support |
| Week 28 | 37 cm | 1.0 kg | Eyes open; brain development accelerates rapidly |
| Week 36 | 47 cm | 2.6 kg | Lungs nearly mature; most systems ready for birth |
| Week 40 | 50 cm | 3.4 kg | Full term; delivery expected within ±2 weeks |
Only 4–5% of babies are born on their exact due date. Research shows that 60% of births occur within one week of the estimated date, and 90% within two weeks. The due date is best understood as the midpoint of a realistic delivery window spanning weeks 38–42, with weeks 37–42 all considered within the full-term range. First-time mothers typically deliver 3–5 days after their due date on average; subsequent pregnancies tend to be earlier. Discuss your individual history and any risk factors with your midwife or obstetrician to understand what your specific timeline looks like.
Frequently Asked Questions
The standard method is Naegele's Rule: add 280 days (40 weeks) to the first day of the last menstrual period, adjusted for cycle length. First-trimester ultrasound, which measures crown-rump length, provides even more accurate dating — typically within 5–7 days at 8–10 weeks and within 7–10 days at 11–14 weeks. Research shows that fewer than 5% of babies are born on their exact due date, 60% arrive within one week, and 90% within two weeks. The due date is statistically best understood as the center of a delivery window spanning roughly weeks 38–42.
Gestational age is measured from the first day of the last menstrual period (LMP) — the standard used by all healthcare providers worldwide. Fetal age (or fertilization age) is measured from the actual moment of conception, which typically occurs approximately 14 days after LMP. A baby described as "40 weeks gestational age" is approximately 38 weeks old from fertilization. This distinction matters because all prenatal milestones, screening windows, and growth charts are standardized to gestational age from LMP, not from conception. When you share your gestational week with your provider, always use the LMP-based count.
Naegele's Rule assumes ovulation on day 14 of a 28-day cycle. For longer cycles, ovulation occurs later, so the due date shifts later by the same number of days. For a 35-day cycle, ovulation is typically around day 21 (7 days later than average), so the due date is approximately 7 days later than the standard Naegele calculation. This calculator automatically adjusts the due date based on your entered cycle length. However, cycle length alone is an approximation — first-trimester ultrasound provides the most accurate dating for women with irregular or non-standard cycles.
The nuchal translucency (NT) scan must be performed between weeks 11 and 14 — this window closes permanently after 14 weeks because the nuchal fold changes appearance. NIPT (non-invasive prenatal testing) is typically offered from week 10 onward. The anatomy scan is best performed between weeks 18 and 22 — earlier is possible but structures are smaller and harder to assess. The glucose tolerance test for gestational diabetes is scheduled at weeks 24–28. Group B Strep swab is done at weeks 35–37. Missing the NT scan window means moving to second-trimester screening options, which have different sensitivity and specificity.
Yes — IVF due dates are calculated by working backward from the embryo's age at transfer. A Day 5 blastocyst is 5 days past fertilization, which is equivalent to 2 weeks and 5 days of gestational age (since gestational age starts 2 weeks before fertilization). So for a Day 5 transfer, the LMP equivalent is 19 days before the transfer date, and the due date is that LMP equivalent plus 280 days. For a Day 3 transfer, the LMP equivalent is 17 days before transfer. IVF due dates are typically more precise than LMP-based estimates because the exact fertilization date is known.
Several symptoms require prompt evaluation rather than waiting for the next scheduled appointment: vaginal bleeding at any stage (light spotting in early pregnancy can be normal but should still be reported); severe or sudden abdominal or pelvic pain; sudden swelling of the face, hands, or feet (especially with headache or visual changes, which may indicate preeclampsia); severe headache unrelieved by paracetamol; blurred vision or visual changes; significant reduction in fetal movement after week 28 (contact your provider if you notice less than 10 movements in 2 hours during a period when the baby is usually active); fever above 38°C; and painful, burning urination (possible urinary tract infection, which is more serious in pregnancy).