Hearts start to beat about 23 days after conception, i.e. about 5 1/2 weeks GA.At this stage, the heart is a simple pulsatile tube. During the next few weeks, the primordial tube folds onto itself and repatterns, forming a divided four chamber pump with filling and emptying sections driving venous blood from the body towards the lungs and from the lungs to the rest of the body.
One of the mysteries of early development has been the signal that causes the heart rate to increase during early development. We now know that this involves a calcium dependent molecular oscillator within heart muscle cells and the appearance and up-regulation of signal receptors on cell surfaces that synchronize contractions. The relationship between heart rate and age in early pregnancy is a basic human developmental feature, it is the same everywhere in the world in normal embryoes. The conventional way to estimate stage in the first trimester is from the length of the embryo. This was first reported by Dr Hugh Robinson in Australia in 1973, one of the enduring pieces of ultrasound research. However, at this point in time, we need to have accurate methods for early in development, when heart rate is the best source of reliable information. |
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Here is our chart for looking up the age of an embryo from heart rate. We use the Doppler to get a precise determination of heart rate from just a few beats. Unfortunately, we cannot use this approach later in development, when external factors take over control of heart rate.
Blood pressure is low in fetuses throughout pregnancy, and oxygen saturation is about equivalent to the norm near the top of Mt Everest. Small hearts only pump a small volume of blood per beat, so they beat rapidly to maintain a circulation adequate for normal development. Fetal heart rates are usually in the 140's in the second and third trimesters, decreasing slightly near the end of pregnancy. Heart rate is related to the size of the heart, which is proportional to body weight. Boys tend to be about a half pound larger than girls at delivery, and, consequently, their heart rates are about 2 beats per minute slower than for girls, on the average, at term.
