Obstetrical Gestation Engine • Naegele's Rule & Milestone Tracker

Pregnancy & Due Date Hub

Q: How does Naegele's rule calculate estimated due date (EDD)?

Naegele's rule calculates the Estimated Due Date (EDD) by taking the first day of your Last Menstrual Period (LMP), adding 1 year, subtracting 3 months, and adding 7 days (or simply adding 280 days / 40 weeks). If your regular menstrual cycle differs from 28 days, an adjustment is made by adding or subtracting the difference between your cycle length and 28 days.

Q: What is the difference between gestational age and fetal conception age?

Gestational age is measured from the first day of your last menstrual period (LMP) and totals approximately 40 weeks (280 days). Fetal age (conceptional age) is measured from the actual moment of ovulation and fertilization, which occurs roughly two weeks later, making fetal age exactly two weeks shorter (38 weeks / 266 days).

Q: How do first-trimester ultrasound scans adjust my estimated due date?

First-trimester crown-rump length (CRL) measurements obtained via ultrasound between 8 and 13 weeks are the gold standard for clinical dating. If the ultrasound gestational age differs from your LMP date by more than 5 to 7 days, American College of Obstetricians and Gynecologists (ACOG) guidelines recommend adjusting the official due date to match the ultrasound measurement.

Q: What are the clinical definitions of early term, full term, late term, and post-term?

ACOG and SMFM classify deliveries into four distinct terms: Early Term (37 weeks 0 days through 38 weeks 6 days), Full Term (39 weeks 0 days through 40 weeks 6 days), Late Term (41 weeks 0 days through 41 weeks 6 days), and Post-Term (42 weeks 0 days and beyond). The optimal window for infant health outcomes is full term.

Calculate your estimated due date, track real-time gestational progress by week and day, explore baby size milestones, and browse our complete 42-week fetal development directory.

First Day of Last Period (LMP)

Average Cycle Length

Estimated Due Date (EDD)

Saturday, March 27, 2027

First Trimester 182 Days Remaining

Gestational Timeline 35.0% Complete (98 of 280 Days)

Current Gestation

14 Weeks, 0 Days

Baby Size

Navel orange

8.7 cm • 43 g

Conception Est.

July 4, 2026

Viability (24 Weeks)

December 5, 2026

Disclaimer: Computational estimates are based on standard clinical algorithms (e.g., Naegele's rule) for educational and planning purposes only. They do not constitute formal medical evaluation, diagnosis, or clinical obstetrics advice.

The Three Trimesters of Pregnancy

A standard 40-week gestation is divided into three clinical trimesters, each bringing unique physiological changes and developmental leaps.

Weeks 1 – 13 Months 1 – 3

First Trimester

Fertilization, blastocyst implantation, and rapid organogenesis. The neural tube closes, tiny limbs sprout, and the heart begins pumping primitive red blood cells.

Milestone: Fetal heartbeat detectable by ultrasound (Weeks 6-8)
Mother: Morning sickness, hormonal shifts, breast tenderness
Baby Size: From poppy seed to Meyer lemon

Explore Week 12 Milestone →

Weeks 14 – 27 Months 4 – 6

Second Trimester

Often termed the "Golden Trimester" as energy rebounds. Baby develops distinct facial features, fingerprints, sucking reflexes, and audible bone structures.

Milestone: Anatomy ultrasound & quickening (kicks) (Weeks 18-20)
Key Marker: Fetal viability threshold at 24 weeks
Baby Size: From navel orange to head of lettuce

Explore Week 20 Milestone →

Weeks 28 – 42 Months 7 – 9

Third Trimester

Fetal weight triples as subcutaneous fat accumulates and surfactant matures lung alveoli. The baby rotates into cephalic (head-down) presentation for labor.

Milestone: Early Term at 37 weeks; Full Term at 39 weeks
Mother: Nesting instincts, pelvic pressure, Braxton Hicks contractions
Baby Size: From eggplant to small pumpkin

Explore Week 40 (Due Date) →

Complete 42-Week Pregnancy Directory

Select any week below to explore comprehensive fetal growth dimensions, maternal anatomy adjustments, symptoms, and exact timeline calculations.

First Trimester: Weeks 1 – 13

(Fertilization to End of Embryonic Period)

Week 1 1st Tri

Microscopic (Fertilization)

< 0.1 mm • < 1 mg