A Frozen Embryo Transfer (FET) is a fertility treatment. Unlike natural conception, where pregnancy dating typically begins from the last menstrual period, determining gestational age after an FET involves a different approach. This article clarifies the methods used to calculate gestational age following an FET.
Understanding Frozen Embryo Transfer
Frozen Embryo Transfer is a common fertility treatment involving the thawing and transfer of an embryo previously created through in vitro fertilization (IVF) and cryopreserved. Embryos are often frozen if a fresh transfer was not suitable, for genetic testing, or for family planning in subsequent cycles. This allows for precise timing of the embryo transfer, optimizing the uterine environment for implantation.
The procedure bypasses natural ovulation and fertilization steps. The uterus is prepared with hormones to create a receptive lining for the thawed embryo. Once transferred, the embryo is expected to implant and continue its development.
Calculating Gestational Age After FET
Calculating gestational age after an FET differs from spontaneous conception because the exact date of fertilization and embryo development is known. Healthcare providers typically backdate the pregnancy to an equivalent Last Menstrual Period (LMP). This ensures consistency with prenatal care protocols and ultrasound measurements.
For a 5-day blastocyst embryo transfer, the equivalent LMP is calculated by subtracting 19 days from the transfer date. For example, if a 5-day embryo was transferred on October 20th, the equivalent LMP would be October 1st (October 20 – 19 days). On the day of transfer, the pregnancy is considered 4 weeks and 3 days gestational age.
When a 3-day cleavage-stage embryo is transferred, the equivalent LMP is determined by subtracting 17 days from the transfer date. If a 3-day embryo was transferred on October 20th, the equivalent LMP would be October 3rd (October 20 – 17 days). This calculation places gestational age at 4 weeks and 0 days on the day of transfer. While some clinics might refer to “weeks post-transfer,” the medical standard converts this to an LMP equivalent for consistency in medical records and care.
Fetal Development Milestones After FET
Once gestational age is accurately calculated after a successful FET, fetal development proceeds along the same timeline as a naturally conceived pregnancy. Early in pregnancy, around 6 to 8 weeks gestational age, a heartbeat can be detected via ultrasound.
By 10 to 12 weeks gestational age, major organ systems begin to develop, and the embryo transitions to being called a fetus. The first trimester is a period of rapid growth and differentiation, laying the groundwork for all future development. Milestones like the development of limbs, facial features, and fetal movement occur at predictable gestational ages. Regular prenatal appointments and ultrasounds monitor these developments, ensuring the fetus grows as expected for its calculated age.
Why Accurate Dating Matters
Accurate determination of gestational age after an FET is important for several reasons. It helps establish a reliable estimated due date, guiding expectations for the baby’s arrival. This dating allows healthcare providers to monitor fetal growth and health, ensuring the baby develops on schedule.
Accurate dating is crucial for timing various prenatal screenings and tests. Tests like non-invasive prenatal testing (NIPT), nuchal translucency scans, and anatomy scans are gestational-age dependent, requiring performance within specific windows for accurate results. Knowing the gestational age helps in decisions about delivery timing, especially with complications or medical indications.