PediAid's Fetal Development guide walks through pregnancy week by week, from the two weeks of follicle maturation before conception through full term (39-41 weeks). Each week card explains what the source calls gestational (menstrual) age versus embryonic/conceptional age: gestational age is measured from the first day of the last menstrual period and is conventionally expressed in completed weeks, so a 36-week-6-day fetus is documented as a 36-week fetus. The guide organizes development by trimester and highlights milestones in organ formation, growth, and lung maturation as they occur.
Gestational (menstrual) age is measured from the first day of the last menstrual period and is what's used clinically because ovulation timing is usually unknown. Conceptional/embryonic age runs from actual fertilization and is roughly two weeks behind gestational age.
The guide flags 24 weeks as the viability threshold, with 23 weeks marked as 'approaching viability threshold.' It also notes approximate survival rates rising through the following weeks (about 87% at 26 weeks, about 94% at 27-28 weeks).
1st trimester is less than 14 weeks 0 days, 2nd trimester runs 14 weeks 0 days through 27 weeks 6 days, and 3rd trimester is 28 weeks 0 days through delivery.
PediAid includes three cited equations: crown-rump length to menstrual age (Hadlock 1992), crown-heel length to gestational age (Fenton), and estimated weight to gestational age (Hadlock 1991).
This is flagged as the critical period for birth defects, since most major organ systems are forming during this window; most such defects have no single known cause or are multifactorial.