Pregnancy Dating: From LMP to Trimester to the Big Day
Naegele's rule, gestational vs fetal age, ultrasound dating, and the milestones week by week
1. Naegele's Rule and Modern Dating
(LMP + 1 year − 3 months + 7 days, Naegele's shortcut)
Naegele's rule, codified in the early 19th century, assumes a 28-day cycle with ovulation on day 14. For non-28-day cycles, you adjust: cycle length minus 28 days is added to or subtracted from the calculated due date. So a 30-day cycle pushes the due date 2 days later.
2. Why Ultrasound Sometimes Overrules LMP
- First trimester (8-13 weeks: ultrasound measures crown-rump length, accurate to ±5-7 days. Best window for establishing dating
- Second trimester (14-27 weeks: head circumference and femur length give ±10-14 days accuracy
- Third trimester (28-40 weeks: ±2-3 weeks accuracy. Growth variance from genetic and nutritional factors dominates
When first-trimester ultrasound substantially disagrees with the LMP-based date (typically by more than 5-7 days), providers generally accept the ultrasound date because it measures fetal development directly
3. Trimester Milestones
T1 (Week 1-13)
Heartbeat visible at 6-7 weeks • Morning sickness peaks • Miscarriage risk drops by end of T1 • NT scan at 11-13 weeks
T2 (Week 14-27)
Energy returns • Anatomy scan 18-22 weeks • Quickening (first movements) 18-22 weeks • Viability at 24 weeks
T3 (Week 28-40)
Rapid growth • Glucose screen 24-28 weeks • Tdap vaccine • Weekly visits after 36 • Term at 37-42 weeks
4. Why the Due Date Is Just an Estimate
Biology is not a perfect clock. Research shows term delivery follows a roughly normal distribution centered on the due date, with most births happening 38-41 weeks and peak frequency around 40 weeks. Provider typical practice is to wait up to 41 weeks for spontaneous labor before considering induction; post-date (42+ weeks) pregnancies are routinely induced due to placental aging concerns.
5. Privacy for Pregnancy Data
Browser-Only Computation
Pregnancy data is among the most sensitive health information. ToolWise's Pregnancy Due Date Calculator runs entirely in your browser — your LMP, conception date, and computed dates never touch our servers or get logged by analytics.
Conclusion
The due date is best understood as a 5-day window or a 2-week"term-month," not an exact day. ToolWise's Pregnancy Due Date Calculator applies Naegele's rule with cycle-length adjustment, gives you the gestational age and trimester, and is best paired with first-trimester ultrasound for the most accurate pregnancy dating.