When a frozen embryo transfer (FET) is performed, a laboratory-created embryo is placed into the uterus, bypassing the natural timeline of conception. Unlike a spontaneous pregnancy, there is no natural last menstrual period (LMP) or reliable ovulation date to mark the beginning of the pregnancy. This precise timing creates a unique challenge in standardizing how far along a person is considered to be. To ensure consistent monitoring and clinical care, a specific calculation is necessary to align the known transfer date with the standard medical pregnancy calendar.
The Concept of Gestational Age in IVF
Gestational age (GA) is the medical standard for tracking a pregnancy, which traditionally calculates the time elapsed from the first day of the last menstrual period, totaling 40 weeks. This standard calculation assumes that conception occurs approximately two weeks after the LMP. In an IVF pregnancy, the exact date of fertilization is known, but medical standards still require reporting GA relative to this 40-week LMP timeline. Clinicians must use a standardized approximation method to translate the known transfer date into this universally accepted model.
The necessity of this translation is for clinical standardization and communication between different medical providers, such as fertility specialists and obstetricians. Since all prenatal screenings, fetal growth milestones, and delivery expectations are based on the 40-week model, the IVF timeline must be artificially adjusted to fit this framework. By aligning the transfer to the standard calendar, healthcare teams can correctly interpret ultrasound measurements and schedule time-sensitive tests.
Calculating the Approximate Last Menstrual Period Date
The first step in determining how far along you are involves establishing a “pseudo-LMP” date, which serves as the foundation for the entire GA calculation. This date is determined by mathematically counting backward from the known embryo transfer date. The calculation is based on the biological fact that in a standard cycle, ovulation occurs 14 days after the LMP, and the embryo is already a specific age when transferred.
When a Day 5 embryo, which has reached the blastocyst stage, is transferred, it is already five days old. Therefore, on the day of the transfer, the pregnancy is considered to be 19 days of gestational age (14 days for the pre-ovulation phase plus the 5 days of the embryo’s development). To establish the pseudo-LMP date, you subtract those 19 days from the date of the transfer. This newly established date then functions as the start of the 40-week pregnancy clock for all medical records and monitoring.
Adjusting the Calculation Based on Embryo Age
The formula for calculating the pseudo-LMP is dependent on the specific biological age of the embryo at the time it was transferred. The most common variation is between a Day 3 embryo and a Day 5 embryo. Since a Day 3 embryo is less mature than a Day 5 blastocyst, the subtraction factor must be adjusted to account for the difference in developmental time.
For a Day 3 cleavage-stage embryo, the gestational age at the point of transfer is 17 days (14 days from the pseudo-LMP plus the 3 days of development). Consequently, the pseudo-LMP date is calculated by subtracting 17 days from the transfer date. The clinical team must know the exact age of the embryo when it was transferred to apply the correct formula for accurate dating. This small, two-day difference in the embryo’s age is important for ensuring that the estimated gestational age matches the embryo’s actual size and development during subsequent ultrasounds.
Determining the Estimated Due Date
Once the pseudo-LMP has been precisely calculated using the adjusted formula, the final step is to determine the Estimated Due Date (EDD). The EDD is set as 40 weeks, or 280 days, from that calculated pseudo-LMP date. This establishes the target date for delivery based on the standard duration of a full-term pregnancy.
Having an accurate EDD is important for scheduling crucial prenatal screenings and monitoring the baby’s growth. For instance, certain blood tests and ultrasound markers are dependent on the correct gestational week to be properly interpreted. The EDD serves as the anchor point for the entire pregnancy timeline, allowing doctors to assess if the fetus is meeting expected growth milestones.