Frozen Embryo Transfer (FET) is a process within assisted reproductive technology where an embryo, cryopreserved from a previous in vitro fertilization (IVF) cycle, is thawed and transferred into the uterus. The goal of FET is to achieve a singleton pregnancy by placing the embryo into a uterine lining that has been carefully prepared to be receptive. Patients undergoing this preparation often wonder about the possibility of conceiving naturally during the process. This biological possibility remains, and though rare, it requires immediate attention from the fertility team.
The Likelihood of Natural Conception During FET Preparation
The chance of natural conception during FET preparation depends heavily on the specific protocol used to prepare the uterine lining. A fully medicated or programmed cycle uses powerful hormones, such as estrogen and progesterone, to suppress the body’s natural hormonal cycle. These medications halt the development of ovarian follicles and prevent ovulation, effectively acting as temporary contraception. In this highly controlled environment, the likelihood of a spontaneous natural pregnancy is extremely low.
However, many patients undergo a modified natural FET cycle, which relies on the body’s ability to develop a follicle and ovulate. The clinic monitors the patient’s natural hormone production and times the embryo transfer to coincide with the body’s natural window of implantation. Since ovulation is allowed to occur, the patient’s underlying fertility is not suppressed. For patients who have residual fertility, sexual intercourse around the time of ovulation creates a small possibility of natural conception.
Pre-Transfer Screening Protocols
Fertility clinics implement rigorous safeguards to prevent the transfer of a frozen embryo into a uterus that is already carrying a spontaneously conceived pregnancy. The primary mechanism for this safeguard is mandatory pre-transfer screening, which involves a blood test to measure the level of the pregnancy hormone, human chorionic gonadotropin (hCG).
This blood test is usually scheduled one to three days before the planned FET date. The timing ensures that any spontaneously conceived pregnancy is detected before the embryo is thawed or transferred. If the hCG blood test returns a positive result, it indicates a natural conception has occurred and results in the immediate cancellation of the FET cycle. This protocol is a necessary measure to protect the patient from the medical risks associated with a dual pregnancy.
Risks Associated with Dual Pregnancy
The clinical concern with a dual pregnancy arises from the introduction of a second embryo into an already pregnant uterus, a rare event called a heterogeneous pregnancy. This scenario presents a high-risk situation for both the mother and the developing fetuses. One significant danger is the increased risk of preterm labor, as the uterus is under strain from carrying two pregnancies. This can result in low birth weight and developmental issues. Statistics show that the majority of twin pregnancies are born before 37 weeks of gestation.
A dual pregnancy also elevates maternal health risks, including a higher incidence of preeclampsia and gestational diabetes. A complication arises if the natural conception is not detected until after the FET preparation medications have started. The high doses of synthetic estrogen and progesterone used to prepare the uterine lining are meant to suppress the body’s natural cycle, and their effect on a newly conceived embryo is not fully understood. If a positive pregnancy test is confirmed, the medical team must immediately stop the FET medications, which can destabilize the uterine environment and potentially increase the risk of miscarriage for the spontaneously conceived embryo.
Immediate Steps if Natural Pregnancy is Suspected
If a patient suspects they may have conceived naturally during the FET preparation cycle, immediate communication with the fertility clinic is paramount. This suspicion may arise from a late period, unexpected spotting, or a positive result on a home pregnancy test. The patient should report all details to their nurse coordinator, including the date of their last menstrual period, dates of sexual activity, and the results of any home tests.
Crucially, the patient should not take any further FET preparation medications, such as estrogen or progesterone, until they have spoken with their physician. The clinic will require an immediate blood draw to confirm the presence and level of the hCG hormone. Pausing the medication is a protective measure. A confirmed natural pregnancy results in the cancellation of the FET cycle and a transition to standard obstetric care.