In vitro fertilization (IVF) involves placing a developing embryo into a patient’s uterus, a procedure known as an embryo transfer. Determining gestational age on the day of transfer can be confusing for patients. Unlike natural conception, standard dating methods are unusable because there is no natural last menstrual period (LMP) or unknown ovulation date to reference. Clinics must use a modified calculation to assign a medically standardized gestational age to the developing pregnancy.
Understanding Traditional Gestational Age
The medical field calculates pregnancy duration using gestational age, which starts the timeline from the first day of the last menstrual period (LMP). A full-term pregnancy is considered 40 weeks, or 280 days, from this date. This means that when conception actually occurs, usually two weeks after the LMP, the patient is already considered two weeks pregnant. Healthcare providers use this system because the LMP is the most reliable fixed point in the reproductive cycle. This 40-week framework allows for standardized tracking of fetal growth and prediction of the estimated due date.
The IVF Timeline Calculation Method
Since an IVF cycle does not rely on a natural LMP, clinics assign a “pseudo-LMP” to align with the standard 40-week model. The date of egg retrieval, when fertilization occurs, serves as the fixed point for this calculation. This date is considered the equivalent of two weeks (14 days) of gestational age, precisely marking the moment of conception. This 14-day starting point is applied regardless of whether the transfer uses a fresh or frozen embryo. The overall gestational age on the day of transfer is determined by adding this 14-day baseline to the actual age of the embryo.
Adjusting the Date for Embryo Development Stage
The final calculation of gestational age depends directly on how many days the embryo was cultured in the laboratory. Embryos are typically transferred at either the Day 3 cleavage stage or the Day 5 blastocyst stage.
Day 3 Transfer
A Day 3 embryo is a developing cluster of four to eight cells that has not yet undergone significant differentiation. If a patient receives a Day 3 transfer, they are considered 2 weeks and 3 days pregnant (14 days plus 3 days of development).
Day 5 Transfer
The Day 5 blastocyst is a more advanced embryo, having developed into a structure containing 70 to 100 cells. These cells have separated into the inner cell mass and the cells that will form the placenta. For a Day 5 blastocyst transfer, the patient is considered 2 weeks and 5 days pregnant on the day of the procedure (14 days plus 5 days of development).