Gestational Age Calculation for Blastocyst Transfer
For a blastocyst embryo transfer, add 19 days (2 weeks and 5 days) to the transfer date to calculate the gestational age on any given date. 1
Standard Dating Method for Assisted Reproduction
In assisted reproductive technology (ART) pregnancies, gestational age is calculated based on the embryo transfer date and the developmental stage of the embryo at transfer, not from the last menstrual period. 1
For a day 5 blastocyst transfer, the embryo is already 5 days post-fertilization at the time of transfer. 1
Since standard gestational age dating assumes fertilization occurs approximately 14 days after the last menstrual period, you must add 14 days (representing the typical follicular phase) plus 5 days (the age of the blastocyst) = 19 days total to the transfer date. 1
Calculation Algorithm
Step 1: Identify the blastocyst transfer date 1
Step 2: Add 19 days to establish the "gestational age starting point" 1
Step 3: Count forward from that adjusted date to your target date 1
Step 4: Express the result as weeks and days (e.g., 8 weeks 3 days) rather than rounded weeks, as this improves accuracy for clinical decision-making 1, 2
Clinical Accuracy Considerations
ART pregnancies with known embryo transfer dates provide the most accurate gestational age estimates possible, with accuracy within approximately ±1.5 days compared to true gestational age. 3
This dating method is more accurate than first-trimester ultrasound crown-rump length (CRL) measurements, which have an error range of 3-8 days. 3
Do not adjust the calculated gestational age based on subsequent ultrasound measurements in ART pregnancies, as the embryo transfer date provides superior accuracy. 2, 3
Important Caveats
This calculation applies specifically to day 5 blastocyst transfers. If a day 6 blastocyst was transferred, add 20 days instead of 19 days. 1
For cleavage-stage embryo transfers (day 2-3), different calculations apply: add 16-17 days for day 2 transfers or 17-18 days for day 3 transfers. 1
Always document both the transfer date and the calculated gestational age in the medical record for clarity in subsequent prenatal care. 2
Use this precise dating for timing of all prenatal screening tests, including aneuploidy screening and anatomic surveys, as accurate gestational age is crucial for proper interpretation of results. 1, 2