An ultrasound is a non-invasive imaging test that uses high-frequency sound waves to create real-time pictures of internal body structures. Ultrasounds help confirm pregnancy, check for potential complications, and determine gestational age. Detecting the fetal heartbeat during an ultrasound is often a significant and reassuring milestone for expectant parents.
Detecting the Early Heartbeat
The earliest a fetal heartbeat can be detected on an ultrasound depends on the type of scan performed. A transvaginal ultrasound, where a device is placed inside the vaginal canal, can typically visualize cardiac activity around 5.5 to 6.5 weeks gestational age. This method provides clearer images in early pregnancy compared to a transabdominal ultrasound. Transabdominal ultrasounds, conducted by moving a transducer over the abdomen, usually detect the heartbeat slightly later, often around 7 weeks.
Before a distinct heartbeat is visible, sonographers look for specific structures that indicate a developing pregnancy. The gestational sac, a fluid-filled sac surrounding the embryo, is typically visible at approximately 5 weeks gestational age. Following this, a yolk sac, a thin-rimmed circular structure within the gestational sac, usually appears around 5.5 weeks and confirms an intrauterine pregnancy. The fetal pole, which is the first visible sign of the developing embryo, can sometimes be seen around 5 to 6 weeks.
Cardiac tissue begins to pulse around 5 to 6 weeks, registering as a flicker on the ultrasound even before the heart is fully formed. This early electrical activity might appear as a rapid flicker, rather than a distinct beating sound, due to the embryo’s small size. The heart itself is not fully developed until about the 10th week of pregnancy.
What the Heartbeat Reveals
Detecting the fetal heartbeat on an ultrasound provides important information about the viability and progression of an early pregnancy. The presence of cardiac activity is considered an early indicator of a healthy, developing pregnancy. It significantly increases the likelihood of the pregnancy continuing. For instance, if a heartbeat is detected at 6 weeks, there is about a 78% chance of the pregnancy continuing, rising to 98% at 8 weeks and 99.4% after 10 weeks.
The fetal heart rate (FHR) is measured during an ultrasound to assess the embryo’s well-being. This is typically done using M-mode, which creates a real-time representation of the heart’s movement, or sometimes pulsed wave Doppler, which also allows the sound to be heard. Normal fetal heart rates vary with gestational age. At around 5 weeks, the embryonic heart rate is similar to the mother’s, about 80-85 beats per minute (bpm).
The heart rate increases rapidly in the very early stages. By 6 weeks, it can be around 110 bpm, and by 9 to 10 weeks, it may reach approximately 170 bpm. After 10 weeks, the rate gradually decreases, typically stabilizing between 110 and 160 bpm for most of the pregnancy. While these are general ranges, variations of 5 to 25 beats per minute within a normal range are common and not usually a cause for concern.
Common Reasons for Undetected Heartbeats
An undetected heartbeat during an early ultrasound can be concerning for expectant parents, but it does not always indicate a problem with the pregnancy. One common reason is a miscalculation of gestational age. This can occur if ovulation happened later than expected or if there are irregular menstrual cycles, meaning the pregnancy is not as far along as initially thought. In such cases, it might simply be too early for the heartbeat to be visible.
The embryo may be smaller than expected for the calculated week, making the heartbeat harder to detect. If no heartbeat is found in an embryo measuring less than 7 mm, a follow-up ultrasound in 7 to 10 days is often recommended. This allows time for further development and clearer visualization.
The position of the uterus can also influence the clarity of the ultrasound image. A retroverted uterus, which tilts backward towards the spine, can make it more challenging to visualize early pregnancy structures, especially with a transabdominal scan. In these instances, a transvaginal ultrasound is often more effective at obtaining clear views. The limitations of early ultrasound equipment or image clarity can also play a role, as very small embryos or subtle cardiac activity may be difficult to capture.