How Due Dates Are Calculated
The standard obstetric convention measures gestational age from the first day of the last menstrual period (LMP), not from conception. Naegele's rule estimates the due date as LMP + 280 days (40 weeks).[1] Conception typically occurs about 14 days after LMP (in a 28-day cycle), so the conception-based formula is conception date + 266 days.
For IVF, the transfer date and embryo age determine the formula:
- Day 3 transfer ??due date = transfer + 263 days
- Day 5 transfer (blastocyst) ??due date = transfer + 261 days
Trimesters
- First trimester ??weeks 1 through 12 (or 13, depending on source). High miscarriage risk; major organ development.
- Second trimester ??weeks 13 through 27. Anatomy scan around week 18??2; viability begins around week 24.
- Third trimester ??weeks 28 through delivery. Rapid growth; full term is 39??0 weeks; "post-term" begins at 42.
Common Milestones
- Week 6?? ??first prenatal appointment, dating ultrasound
- Week 10??3 ??first-trimester screening (NT scan, NIPT)
- Week 15??0 ??quad screen / amniocentesis if indicated
- Week 18??2 ??anatomy ultrasound
- Week 24??8 ??glucose challenge test (gestational diabetes screen)
- Week 27??6 ??Tdap booster recommended
- Week 35??7 ??Group B Strep (GBS) culture
- Week 37 ??early term
- Week 39??0 ??full term
- Week 41+ ??late term; medical induction often discussed
- Week 42+ ??post term
Accuracy of LMP Due Dates
LMP-based dating assumes a regular 28-day cycle and ovulation on day 14. Real cycles vary; longer cycles push ovulation (and conception) later, which moves the due date later. First-trimester ultrasound measurements of crown-rump length are usually used to refine the due date and are considered more accurate when LMP is uncertain.[2]
References
[1] Wikipedia, "Naegele's rule" (CC BY-SA 4.0). [2] American College of Obstetricians and Gynecologists ??Method for Estimating Due Date. acog.org