Pregnancy Calculator Guide: Due Date & Trimester Tracking孕期计算器指南：预产期与孕周追踪
From the moment a pregnancy is confirmed, two questions dominate: "When is the baby due?" and "How far along am I?" Both have precise medical answers, both rely on a 280-day convention that doesn't match real conception dates for most people, and both get refined by ultrasound. This guide explains how doctors actually date pregnancies, the math behind the calculators, and what the trimester cutoffs mean.
Why Pregnancy Is Dated From Your Last Period
Pregnancy dating has a quirk: the clock starts two weeks before you actually conceived. The first day of your last menstrual period (LMP) is treated as "day 1" of pregnancy, even though ovulation and conception happen around day 14. This sounds odd, but it has a reason: most women know their LMP date exactly, while the moment of conception is unknowable without specialized testing.
This convention produces a "gestational age" that is always two weeks ahead of the actual embryonic age. At "8 weeks pregnant," the embryo has only been developing for about 6 weeks.
Naegele's Rule — The Due Date Formula
The classic due date calculation, dating to the 1800s but still standard:
Equivalent to LMP + 1 year − 3 months + 7 days (Naegele's original form).
Worked example: if your last period started on January 1, 2026:
Due Date = Jan 1 + 280 days = October 8, 2026
Naegele's rule assumes a 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, the calculation needs adjustment (see below).
Adjusting for Cycle Length
If your cycle isn't 28 days, Naegele's rule needs a correction:
A 35-day cycle adds 7 days. A 26-day cycle subtracts 2 days.
The reasoning: with a 35-day cycle, you ovulate about a week later than the 28-day default, so conception is later, so the due date is later.
How Ultrasound Refines the Date
LMP dating is accurate to ±1 week for women with regular 28-day cycles. For everyone else, first-trimester ultrasound is more accurate — typically within ±3–5 days.
How ultrasound dating works:
- 6–9 weeks: crown-rump length (CRL) is measured. This gives gestational age accurate to ±5 days.
- 9–14 weeks: CRL remains the gold standard. Accuracy similar.
- 14+ weeks: head circumference, femur length, and abdominal circumference are combined. Accuracy drops to ±1–2 weeks.
Doctors use the ultrasound date if it differs from the LMP date by more than 5 days in the first trimester. After 14 weeks, ultrasound can't reliably overturn LMP dating.
The Three Trimesters
Pregnancy is divided into three trimesters, each roughly 13 weeks:
| Trimester | Weeks (LMP-based) | Major developments |
|---|---|---|
| First | 1–13 | Organ formation, morning sickness, first heartbeat detectable at ~6 weeks |
| Second | 14–27 | Movement felt (~20 weeks), anatomy scan at 18–22 weeks, viability threshold at 24 weeks |
| Third | 28–40+ | Rapid growth, Braxton Hicks contractions, full-term at 37 weeks, late-term at 41 weeks |
"Full term" is now defined as 39–40 weeks, "early term" as 37–38, "preterm" as under 37, and "late term" as 41+. Only about 4% of babies arrive on their exact due date — most come within two weeks of it.
If You Know Your Conception Date
For IVF pregnancies or known conception dates, the calculation is more direct:
266 days = 38 weeks of actual fetal development. Add 14 days to convert to LMP-based gestational age.
This gives a more biologically accurate date because it doesn't rely on the LMP convention.
Key Milestone Dates
Once you have your due date, several milestones become predictable:
| Milestone | When (from LMP) |
|---|---|
| Pregnancy detectable on home test | ~4 weeks |
| First prenatal appointment | 8–10 weeks |
| Heartbeat detectable on ultrasound | ~6–7 weeks |
| First trimester ends | 13 weeks |
| Anatomy scan | 18–22 weeks |
| Viability (survival outside womb with care) | 24 weeks |
| Third trimester starts | 28 weeks |
| Full-term | 39 weeks |
| Due date | 40 weeks (280 days from LMP) |
| Late-term | 41 weeks |
| Post-term | 42+ weeks |
An online calculator gives you a starting point. Your doctor will refine the date based on ultrasound and clinical findings — and the date can still change. Always defer to your provider for medical decisions about timing.
Try It
Use the Pregnancy Calculator to:
- Compute your due date from LMP or known conception date.
- See your current gestational age and how far along you are.
- Track which trimester you're in and the milestones ahead.
The calculator adjusts for cycle length and shows all key milestone dates from positive test to post-term.