Pregnancy dating after a frozen embryo transfer (FET) often presents unique questions for expectant parents. Unlike natural conception, where dating begins from the last menstrual period, an FET involves precise medical timing. Understanding how pregnancy weeks are determined following an FET provides clarity on your journey.
The Distinct Approach to FET Pregnancy Dating
Pregnancy dating after a frozen embryo transfer differs significantly from natural conception, as there is no traditional last menstrual period (LMP) to use as a starting point. In an FET cycle, medical professionals control the uterine environment with hormones, preparing it to receive the embryo. This controlled process means that ovulation, if it occurs naturally, is not the primary marker for dating.
Instead, pregnancy is dated from a “mock” or “estimated” LMP, a calculated date established by the fertility clinic. This mock LMP accounts for the precise timing of the embryo transfer and the embryo’s developmental stage. This method is necessary because there is no spontaneous ovulation and the embryo is directly placed.
How to Calculate Your Pregnancy Week
Healthcare providers establish a “mock” last menstrual period (LMP) to date the pregnancy, aligning with standard 40-week gestational dating practices. For a 5-day blastocyst transfer, the mock LMP is typically set 19 days before the actual transfer date. This means that on the day of a 5-day blastocyst transfer, you are considered 2 weeks and 5 days pregnant.
Similarly, for a 3-day embryo transfer, the mock LMP is set 17 days before the transfer date. On the day of a 3-day embryo transfer, the pregnancy is considered 2 weeks and 3 days along. To find your current week, count forward from this mock LMP or add the days/weeks since your transfer to the initial gestational age. For example, if you had a 5-day blastocyst transfer and it has been 7 days, you would be 3 weeks and 5 days pregnant (2 weeks, 5 days + 1 week). Always confirm calculations with your fertility clinic for accurate, individualized dating.
Important Milestones in an FET Pregnancy
After the embryo transfer, the first milestone is confirming pregnancy through a blood test. This involves measuring human chorionic gonadotropin (hCG) levels, typically 9 to 14 days post-transfer. A positive hCG result indicates successful implantation. Monitoring hCG levels over a few days provides an early indicator of a developing pregnancy.
The next step is the first ultrasound scan, usually scheduled around 6 to 7 weeks of gestation, or three to five weeks after the embryo transfer. This initial scan aims to confirm the presence and location of the gestational sac, identify the yolk sac, and visualize the fetal pole. A fetal heartbeat can often be detected during this scan, which is a sign of viability.
Following confirmation of pregnancy and fetal viability, patients typically transition from their fertility clinic’s care to a regular obstetrician (OB/GYN) for ongoing prenatal care. This transition often occurs between 8 to 10 weeks of pregnancy, once the pregnancy is stable and initial monitoring by the fertility clinic is complete. Your fertility clinic will provide medical records to your new obstetric care provider to ensure seamless care.