Is gestational age (the age of a pregnancy, calculated from the first day of the last menstrual period) always older than embryonic age (the age of the developing embryo, calculated from the date of fertilization)?

Is Gestational Age Always Older Than Embryonic Age?

Yes, gestational age is always approximately 2 weeks older than embryonic age because gestational age is calculated from the first day of the last menstrual period, which occurs roughly 2 weeks before ovulation and fertilization. 1

Understanding the Terminology

Gestational age (GA) refers to the duration of pregnancy measured from the first day of the last menstrual period (LMP), expressed in completed weeks and days. 2 This is the standard convention used in obstetrics for dating pregnancies and timing clinical interventions. 2

Embryonic age (also called "conceptual age" or "fetal age") measures the actual time since fertilization/conception, which typically occurs at ovulation—approximately 14 days after the start of the last menstrual period. 2, 1

The Mathematical Relationship

• Gestational age = Embryonic age + 2 weeks (approximately) 1
• The embryonic period of 4-8 weeks post-ovulation corresponds to 6-10 weeks gestational age from LMP 1
• This 2-week difference is built into the dating system and remains constant throughout pregnancy 1

Clinical Implications

The distinction matters most during organogenesis:

• Organogenesis occurs at embryonic age 4-8 weeks, which equals 6-10 weeks GA—this represents the period of highest teratogenic risk 1
• The embryonic period extends until 10 weeks 6 days GA, after which the conceptus becomes a fetus at 11 weeks 0 days GA 2, 1

Important Caveats

In assisted reproductive technology (ART) pregnancies, the calculation differs because the exact fertilization date is known:

• For day 5 blastocyst transfers, add 19 days to the transfer date to calculate gestational age 3
• This provides superior accuracy compared to LMP-based dating 3

The assumption of a 28-day cycle with day 14 ovulation may not apply to all women, which can introduce errors in LMP-based dating. 4 Women with longer menstrual cycles may have their gestational age overestimated by the LMP method, leading to misclassification of postterm deliveries. 4

First-trimester ultrasound (crown-rump length measurement) provides more accurate gestational age estimation than LMP dating, with accuracy within ±5-7 days, and should be used to adjust the estimated due date when discrepancies exceed 5 days. 5