From the Guidelines
A fetus develops a heartbeat around 6 weeks of pregnancy, as indicated by cardiac activity visualized on transvaginal ultrasound. At this early stage, what develops is actually a cardiac tube that will eventually become the heart, and it begins to pulse and circulate blood 1. The heartbeat can sometimes be detected by transvaginal ultrasound as early as 5.5-6 weeks, though it may not be audible until around 10-12 weeks when using a Doppler device in a clinical setting. It's essential to understand that this early cardiac activity is different from a fully formed heart, which continues to develop throughout pregnancy.
Some key points to consider:
- The four chambers of the heart form between weeks 5-8, with the heart becoming fully formed by around week 10 1.
- This early development is part of the critical period of organogenesis when many major structures begin to form.
- If you're tracking pregnancy milestones, remember that these timing estimates are calculated from the first day of the last menstrual period, not from the day of conception.
- The term "cardiac activity" is preferred over "heartbeat" to describe the rhythmic pulsations in the embryo/fetus, as the heart is not yet fully formed 1.
In clinical practice, it's crucial to prioritize the most recent and highest-quality evidence when assessing fetal development and cardiac activity. The American Journal of Obstetrics and Gynecology provides guidance on the lexicon for first-trimester ultrasound, emphasizing the importance of accurate terminology and avoiding misleading phrases 1. By following these recommendations and staying up-to-date with the latest research, healthcare providers can ensure the best possible outcomes for their patients.
From the Research
Fetal Heartbeat Development
- The development of a fetal heartbeat is a significant milestone in pregnancy, and its detection is crucial for monitoring fetal health.
- According to a study published in 1994 2, embryonic cardiac activity is first apparent at day 34 of gestation and should be visible by day 37 in normal pregnancies.
- Another study from 1988 3 found that fetal heart motion can be detected at around 46.9 days from the last menstrual period using transvaginal ultrasonography.
Detection of Fetal Heartbeat
- Transvaginal Doppler auscultation is a sensitive method for detecting fetal heart rate, especially in the first trimester of pregnancy 4.
- A study from 1995 5 used transvaginal ultrasound to longitudinally describe the normal embryonic development in vivo and found that the heart rate increases rapidly to a mean of 175 beats per minute at the beginning of week 9.
- The earliest fetal heart rate detected transvaginally was at 6 weeks 0 days of gestation, compared to 7 weeks 0 days transabdominally 4.
Fetal Heart Rate and Gestational Age
- The mean heart diameter was found to be 22% of the crown-rump length at 7 weeks, 17% at 9 weeks, and only 13% at 12 weeks 5.
- A study from 2021 6 found that an intrapartum fetal heart rate baseline ranging between 150 and 160 bpm at or after 40 weeks is associated with a higher incidence of labor complications.
- The fetal heart rate rises from 94 to 166 beats/min from days 34 to 56 of gestation 2.