In assisted reproductive technology (ART), an embryo is a fertilized egg that has begun to develop outside the human body, typically in a laboratory setting. These embryos are often created through procedures like in vitro fertilization (IVF), where sperm and eggs are combined for fertilization. Embryo freezing, also known as cryopreservation, preserves these embryos for future use. This technique allows individuals and couples to store embryos for later pregnancy attempts.
Embryo Development and Heartbeat Formation
The cardiovascular system is among the first organ systems to begin functioning. The preliminary heart structures begin spontaneously pulsing around week five of pregnancy, or about 21 to 23 days after fertilization. This initial activity is often referred to as “cardiac activity” rather than a fully developed heartbeat, as the heart is not yet structurally complete.
At this early point, the developing heart resembles a tube that twists and divides to form the future four chambers and valves. This electrical activity, detectable by ultrasound, indicates that development is progressing. While the characteristic “lub-dub” sound of a mature heart, produced by the opening and closing of valves, is not present at this stage, the rhythmic contractions signal the beginning of the circulatory system.
The Dormant State of Frozen Embryos
A frozen embryo does not possess a heartbeat because cryopreservation suspends all biological activity. Cryopreservation involves cooling biological samples to very low temperatures, typically to -196°C in liquid nitrogen. This extreme cold halts all metabolic processes within the embryonic cells, including growth, development, and cellular functions.
During cryopreservation, cryoprotectants are introduced to the embryo. These substances, such as glycerol or dimethyl sulfoxide (DMSO), prevent the formation of damaging ice crystals inside the cells. By replacing water within the cells and increasing solute concentration, cryoprotectants allow the embryo to enter a state of suspended animation. Therefore, while biologically viable, the embryo is not actively developing or exhibiting signs of life, such as a heartbeat, in this frozen state.
Detecting a Heartbeat After Thawing and Transfer
Once a frozen embryo is thawed, it rehydrates as cryoprotectants are removed, reactivating its biological activity. If the embryo survives thawing and successfully implants into the uterine lining after transfer, its development can resume. The first ultrasound after an IVF embryo transfer is typically scheduled around the sixth or seventh week of pregnancy, approximately three to five weeks post-transfer.
At this stage, a healthcare provider can usually detect cardiac activity using a transvaginal ultrasound. This early detection is a positive sign that the pregnancy is progressing. The embryonic heart rate at six weeks can range from 90 to 110 beats per minute and rapidly increases in the following weeks, often reaching 150 to 170 beats per minute by nine or ten weeks.