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An unexpected beginning: Why are six-week-old embryos already able to move?
Embryonic movement refers to the movement caused by the activity of the embryo's own muscles. According to research, motor activity begins in late embryological stages and changes its nature with development. As early as six weeks old, embryos can display some basic movements that are not reflexes but originate from spontaneous nerve impulses generated in the spinal cord. As the nervous system matures, muscles are able to respond to stimuli.
Generally speaking, fetal movements can be divided into guided movements or spontaneous movements, while spontaneous movements can be triggered by the spinal cord or the brain.
It is worth noting that regarding the definition of movement, some studies believe that voluntary movements do not occur until after birth, while other studies indicate that purposeful movements actually begin to occur months before. Through three-dimensional ultrasound technology, we can observe the movement of the fetus. These images are called "four-dimensional ultrasound."
Movement in development
First trimester
Embryonic stage
Before the fetal stage begins, six-week-old human embryos are already able to arch their backs and necks. By seven weeks, limb movement can be detected through ultrasound.
Fetal stage
The parts of the fetal brain that control movement need to wait until the end of the second trimester and the early part of the third trimester to fully form. At birth, control of movement remains limited, while purposeful voluntary movement gradually develops over a period of more than a decade.
According to an overview provided by the Royal Society of Medicine of Scotland, purposeful movements begin at approximately 18 weeks, gradually replacing reflex movements, while purposeful voluntary movements develop further after birth.
In early locomotion, the limbs move together; by the ninth week, as control neurons develop in the spinal cord, the limbs begin to move independently. At the 11th week, the fetus can open its mouth and suck its fingers; at the 12th week, it begins to swallow amniotic fluid. In addition to sideways bending of the head, fetal movements in the early stages include systemic movements and startle responses.
Second trimester
By the 12th week, the fetus can kick its legs and flex its toes, and even grab its own feet or scratch itself with its nails. Fetuses can also respond to skin touch. Around the 12th week, the diaphragm of the carcass will move up and down to show that the fetus is "breathing", but this movement will disappear by the 16th week and will not resume until the third trimester.
The mother usually first feels fetal movement, called "rapid movements," in the fifth month. By this time, limb movements have become more complex, including flexion of joints and ribs. Women who have given birth to children are more sensitive to fetal movements because their uterine muscles are more relaxed, and they may sometimes be able to feel fetal movements as early as the 14th week.
When the fetus grows to the 21st week, it begins to form a regular movement pattern; by the 24th week, about half of the fetuses will have a startle reflex, and it will be fully developed by the 28th week. At this time, because the fetus has become very large, movement is restricted, and there is limited room to kick or change body position.
Third trimester
In late pregnancy, a complex movement called "stepping" begins to develop. This movement involves a circular "cycling" movement of the legs, which helps the fetus prepare for birth in a head-down position.
Fetuses between 14 and 18 weeks of age exhibit distinct circadian rhythms, which can be detected through fetal electrocardiography and measurement of motor activity.
The fetal activity is not consistent with the mother's activity time; the fetal most active time is usually 9 am to 2 pm, and 7 pm to 4 am. During the last four to six weeks before birth, most kicking and poking occurs when the fetus is in light sleep.
Monitor fetal movement
After sensing rapid movements, pregnant women may choose to count the number and types of fetal movements, a method informally known as "kick counting." The American Pregnancy Association points out that the benefits of kick counting include promoting the connection between pregnant women and their babies and reducing the risk of stillbirth; kick counting is recommended especially during high-risk pregnancies. However, instructing women to monitor fetal movement may be associated with increased maternal anxiety.
70% of pregnancies with just one episode of reduced fetal movement are uncomplicated. In the UK, the Royal College of Obstetricians and Gynecologists recommends that pregnant women contact their midwives immediately if they think their fetus is no longer moving or has reduced activity and they are more than 28 weeks pregnant. Do not wait until the next day to contact the hospital or midwife.
Under such circumstances, have you ever thought about what kind of message the fetal movement conveys?
