A baby’s heart starts developing very early in pregnancy. The first coordinated heart activity usually begins around the fourth week after fertilization, which is about 5 to 6 weeks of pregnancy when counted from the last menstrual period.
This does not mean the heart is fully formed at that time. Early on, it is more like a small tube that is starting to contract and move blood, not a fully developed four-chambered heart. The heart continues to grow and become more complex over the following weeks.
On ultrasound, cardiac activity is often seen around 6 weeks of pregnancy, especially with a transvaginal ultrasound. Sometimes it may not be visible right away because of dating differences, the type of ultrasound used, or how early the pregnancy is.
If there is uncertainty about whether cardiac activity is present, the usual next step is repeat ultrasound rather than assuming the worst from one early scan. Anyone with bleeding, strong cramping, or concerns about early pregnancy should contact an OB-GYN or pregnancy care provider for proper evaluation.