Pregnancy Calculator
Where you are in the journey, week-by-week.
Key Milestones
| Week | Date | Milestone |
|---|
How Pregnancy Weeks Are Counted
By long-standing clinical convention, pregnancy is counted from the first day of the last menstrual period (LMP), even though conception happens roughly 2 weeks later. So at "4 weeks pregnant," the embryo has actually existed for about 2 weeks. This 280-day (40-week) count is the global obstetric standard, used by the American College of Obstetricians and Gynecologists (ACOG) and equivalent bodies worldwide. The Wikipedia article on gestational age covers the historical reasons (it predates accurate ovulation tracking).
The Three Trimesters
| Trimester | Weeks | Key Developments |
|---|---|---|
| First | 1-13 | Heart begins beating (~6 weeks), organs form, highest miscarriage risk |
| Second | 14-27 | "Honeymoon trimester" — energy returns, anatomy scan, gender identification |
| Third | 28-40+ | Rapid weight gain, lung maturation, fetal positioning |
Reality of full-term delivery timing
Despite the precision of the 280-day calculation, actual delivery dates spread widely. A large analysis published in Human Reproduction (Jukic et al., 2013) found:
| Outcome | Probability |
|---|---|
| Delivered on exact due date | 4-5% |
| Delivered within ±1 week of due date | ~60% |
| Delivered within ±2 weeks of due date | ~85% |
| Pre-term (before 37 weeks) | ~10% |
| Post-term (after 42 weeks) | ~5-7% |
This is why most providers describe the calculated date as an "estimated" due date or EDD. Plan for a window of roughly ±2 weeks. First-time mothers tend to deliver slightly later than the EDD; subsequent pregnancies often arrive earlier.
Important Milestone Weeks
This calculator highlights weeks commonly used in prenatal care for screenings, anatomy scans and viability checkpoints. Specific appointment timing varies by provider, country, and individual circumstances — these are typical North American/European benchmarks:
- Week 8-10: First prenatal visit, often with dating ultrasound
- Week 10-13: NIPT (non-invasive prenatal testing) and first-trimester screening
- Week 12-13: End of first trimester; risk of miscarriage drops sharply
- Week 18-22: Anatomy ultrasound (sometimes called "20-week scan")
- Week 24: Viability threshold — with NICU care, ~50% survival if born here
- Week 26-28: Glucose tolerance test for gestational diabetes
- Week 28+: Third trimester begins; visits become more frequent
- Week 35-37: Group B strep (GBS) screening
- Week 37: Considered "full term"; baby's lungs are typically mature
- Week 40: Estimated due date
- Week 41-42: Provider may discuss induction
Pregnancy Health Quick Facts
Reliable information sources for pregnant individuals:
- CDC Pregnancy Resources — vaccinations, infections, travel safety
- ACOG Pregnancy Page — clinical guidelines from the leading US obstetric body
- UK NHS Pregnancy Guide — evidence-based pregnancy information
- WHO Maternal Health — global guidelines and standards
Frequently Asked Questions
Why is pregnancy counted from LMP if I didn't conceive then?
The LMP is something women can usually identify; the exact conception date typically isn't known. LMP-based dating has been the obstetric standard for over a century. Modern first-trimester ultrasounds (especially CRL — crown-rump length — between 7-13 weeks) are more accurate and override LMP if they disagree by more than 7 days.
How accurate is this calculator?
Very accurate if your menstrual cycle is regular at 28 days and you know your LMP exactly. For irregular cycles or unknown LMP, an early dating ultrasound is more reliable.
What's "full term"?
37-42 weeks is "term." 37-38w6d is "early term," 39-40w6d "full term," 41-41w6d "late term," 42+ "post term." Earlier than 37 is "preterm." These definitions matter for newborn risk classification.
Are weekly app updates accurate?
Pregnancy apps usually give a daily-by-daily fetal development description that's roughly accurate but oversimplified. Real fetal development varies by ±1-2 weeks between individuals. Use them for fun and rough orientation, not medical decisions.
Should I share my due date publicly?
Personal choice — but be aware that you'll likely deliver up to 2 weeks before or after, and answering "have you had the baby yet?" for the entire last month gets old fast. Many people share a "due month" or "due in early/mid/late [month]" instead of a specific date.