When pregnancy is achieved through In Vitro Fertilization (IVF), tracking gestational age differs significantly from natural conception. Unlike spontaneous pregnancies where fertilization timing is uncertain, IVF provides a precise, known date for when the egg and sperm were combined. This certainty allows for a highly accurate gestational timeline, based on the developmental age of the embryo at the time of transfer.
The Standard Calculation Method
In traditional obstetrics, gestational age is calculated using the date of the Last Menstrual Period, or LMP. This method adds 40 weeks to the first day of the last period, which assumes that conception occurred approximately two weeks later. Therefore, a pregnancy is considered two weeks along even before fertilization has taken place, creating a standard gestational age that is two weeks ahead of the actual fetal age.
This standard approach is often unreliable or impossible to use for many IVF patients. Women undergoing IVF may have irregular cycles or use medicated cycles that suppress their natural period. If a person uses a donor egg or a gestational carrier, their LMP is irrelevant to the embryo’s age. The IVF method bypasses these variables by using the known dates of fertilization and transfer as fixed reference points.
Calculating Weeks Based on Fresh IVF Cycles
For a fresh IVF cycle, the calculation for gestational age is based on the date of the egg retrieval, which is equivalent to the day of fertilization. The retrieval date is systematically counted as 2 weeks and 0 days (14 days) of gestation, aligning the IVF timeline with the traditional LMP-based timeline. This is done so that medical records and ultrasounds remain consistent with standard obstetric dating practices.
To determine the current gestational age, the clinic uses the transfer date and the embryo’s age at transfer. For a common 3-day embryo transfer, the transfer date is considered 2 weeks and 3 days pregnant, as the three days of lab development are added to the initial two weeks. To find the equivalent LMP date, which doctors use for dating, you would subtract 17 days from the date of the 3-day embryo transfer.
If a 5-day blastocyst was transferred, the transfer date is counted as 2 weeks and 5 days gestation. To find the calculated LMP equivalent, you would subtract 19 days from the 5-day blastocyst transfer date. This simple subtraction “back-dates” the pregnancy to a theoretical LMP, allowing for seamless integration into existing medical software and due date calculations. This fixed dating ensures the gestational age is known with precision.
Adjustments for Frozen Embryo Transfers
Frozen Embryo Transfer (FET) cycles use the same fundamental principle as fresh cycles, but calculations rely on different hormonal milestones. In a medicated FET cycle, the start date of progesterone supplementation replaces the egg retrieval date as the primary marker. Progesterone prepares the uterine lining for implantation, mimicking the natural process that occurs after ovulation.
For a common 5-day blastocyst FET, the transfer is typically scheduled five days after the start of progesterone. This means the transfer date is counted as 2 weeks and 5 days pregnant, just like in a fresh 5-day transfer. The date the patient started progesterone is considered the equivalent of “ovulation,” and the date two weeks prior to progesterone start is often used to establish the LMP equivalent.
If a fully medicated cycle is used, the clinic determines the LMP equivalent by subtracting 19 days from the day the 5-day blastocyst was transferred. This “back-dating” ensures the gestational age reflects the embryo’s development and the uterine environment’s receptivity window, regardless of when the embryo was originally created. The specific age of the embryo is precisely documented by the lab and clinic for calculation purposes.
Determining the Estimated Due Date
The precision of IVF calculations leads to an accurate Estimated Due Date (EDD) compared to the traditional LMP method. Once the gestational age is established using the transfer date and embryo age, the EDD is determined by calculating the date 40 weeks from the calculated “LMP equivalent.” A full-term pregnancy is considered 280 days, or 40 weeks, from this theoretical starting point.
Alternatively, the EDD can be calculated by adding 266 days, or 38 weeks, to the date of fertilization (egg retrieval in a fresh cycle). Because the date of fertilization is a known variable in IVF, this 38-week calculation is considered the most biologically accurate method for determining the baby’s arrival. While the EDD is a reliable estimate, it is important to remember that few babies are born exactly on their due date.