Development of the human heart inside the womb is fascinating and complex. As a preborn baby develops, intricate and miraculous happenings occur, from head to toe, through every stage of pregnancy. The heart, one of the first organs to form, begins beating just 22 days after conception.
Pregnancy is divided into three trimesters. The first encompasses up to and including 12 weeks’ gestation. The second trimester is from 13 to 28 weeks, and the third encompasses 29 to birth, generally at or near 40 weeks.
During all three trimesters, a preborn baby’s heart and circulation system continue to develop, starting at the beginning of a woman’s pregnancy.
First Trimester
A baby undergoes remarkable growth and development from conception to birth, with each trimester bringing critical changes—from a rapidly forming heart in the first trimester to fully matured lungs and readiness for life outside the womb in the final weeks—all a miraculous work of our Creator God.
Formation of the Embryonic Heart
During the first few weeks of pregnancy, the embryonic heart begins to form. It starts as two tubes that fuse together and fold during the sixth week of pregnancy. The fusion forms an s-curve which will later become the atria and the ventricles. This is called ‘looping.’
The heart begins to pulse at about 110 beats per minute during the fifth week, and by the following week, the preborn baby’s heartbeat can often be detected via transvaginal ultrasound.
The central nervous system also begins forming during this time. The brain and spinal cord are developing from what is known as the neural tube.
Weeks 7 and 8
Rapid development of the heart occurs during these weeks. According to the Charlotte Lozier Institute, the research and educational arm of the Susan B. Anthony Pro-Life America organization, during the seventh week “two sections become the atria,” and the next week, “the ventricles form.” This creates the four chambers of the heart.
Additionally, the heart valves develop and the circulatory system kicks into gear. According to the Children’s Hospital of Philadelphia, a preborn baby’s circulatory system uses three shunts. These are “small passages that direct blood that needs to be oxygenated.” The shunts’ purpose are to bypass certain body parts, especially the developing lungs and liver. A preborn baby receives oxygenated blood from the mother via the umbilical cord.
The presence of these three shunts causes the circulation of a preborn baby’s heart to differ drastically from an adult heart even though both have four chambers.
How the Shunts Work
Children’s Hospital of Philadelphia describes the workings of the shunts this way:
“The shunts that bypass the lungs are called the foramen ovale, which moves blood from the right atrium of the heart to the left atrium, and the ductus arteriosus, which moves blood from the pulmonary artery to the aorta.
Oxygen and nutrients from the mother’s blood are transferred across the placenta to the fetus. The enriched blood flows through the umbilical cord to the liver and splits into three branches. The blood then reaches the inferior vena cava, a major vein connected to the heart. Most of this blood is sent through the ductus venosus, also a shunt that passes highly oxygenated blood through the liver to the inferior vena cava and then to the right atrium of the heart. A small amount of this blood goes directly to the liver to give it the oxygen and nutrients it needs.
Waste products from the fetal blood are transferred back across the placenta to the mother’s blood.”
Weeks 9 – 12
The nineth week of gestation brings even more change, not only to the baby’s heart, but to other important organs and tissues. Tiny arms and legs form as do fingers and toes (although at first, webbing appears).
The cortex, which is the thinking and sensing part of the human brain, doubles in size, and brain cells, known as neurons, connect to one another.
Additionally, the inner and outer ears begin to form, along with knees and elbows. All four chambers of the heart also continue to develop during this important time of growth, and the preborn baby’s heart rate increases to 170 beats per minute.
At this time, the preborn baby enters the fetus stage and is referred to as such by the majority in the medical community until birth.
In the 10th week, the digestive system and kidneys begin working. Fingers and toes can wiggle. The eyes have a lens, cornea, and retina, and the front and back of the eyeball fuse together.
It’s at this point that a preborn baby’s heart reaches its final shape, “with two atria, two ventricles, and circulatory blood vessels.” However, the blood vessels “mostly bypass the liver and lungs” so that the baby in the womb receives oxygenated blood from the umbilical cord.
During the 11th week, a number of changes occur. Red blood cells begin to form in the liver, and genitalia begins developing, according to the Mayo Clinic. The baby’s brain is developing left and right hemispheres. “The left side is responsible for logic, while the right will help baby process language,” according to The Bump.
At week 12, all organs, limbs, bones, and muscles “are present,” according to the Cleveland Clinic. Additionally, the preborn baby’s face takes on a more developed profile, and the intestines are in the abdomen.
Additionally, a preborn baby’s circulatory, digestive, and urinary systems are working, and its liver produces bile. The heart and circulatory system pumps an average of six quarts of blood per day, and the preborn baby’s heart rate is around 167 beats per minute.
Second Trimester
During the second trimester, a preborn baby undergoes critical heart maturation, develops sensory abilities like hearing and response to light, forms essential structures such as fingerprints and capillaries, and by 28 weeks, has a maturing nervous system and lungs capable of supporting life outside the womb with medical assistance.
Weeks 13 – 20
Researchers discovered this is a critical time in the maturation of a preborn baby’s heart. The study, led by Dr Eleftheria Pervolaraki from the University of Leeds, used MRI technology to determine a four-day window of critical development in the 17th week of pregnancy. Muscle tissues of the heart organize rapidly around this time. According to the researchers, “Cardiac fibers are laid down to form the helix shape of the heart within which the four chambers of the heart form. Without this essential architecture in place, the fetal heart cannot survive outside the womb.”
Proteins aid in the maturation of the baby’s heart.
“By the second trimester, the heart of the [preborn baby] is fully formed,” according to Medical News Today.
Heart tones can be picked up on a doppler between the 12th and 15th week of pregnancy and can be heard by the doctor with a stethoscope by the 20th week.
During weeks 16 and 17, the preborn baby develops lips and ears and begins to put on fat. Additionally, its heart pumps about 100 pints of blood each day.
At 18 weeks, a preborn baby can begin to hear; the eyes start to face forward, and its digestive system starts working.
At 19 weeks, a protective coating called vernix caseosa covers the preborn baby’s delicate skin, and by 20 weeks, a mother usually begins feeling her preborn baby move or kick, which is called quickening.
Weeks 21 – 28
All tissues and organs, including the lungs and heart, continue growing and developing. Some of the tiniest components of the human body, such as capillaries (blood vessels), form and begin to function around 21 weeks.
Also at 21 weeks, a fine, downy hair called lanugo covers the preborn baby, and the sucking reflex develops. The arms and legs are in better proportion, and cartilage throughout the body begins turning to bone. Also, bone marrow forms, helping to produce blood cells.
Neurons are now connected between the developing baby’s brain and muscles. Heart rates between 110 and 160 are considered normal.
Preborn babies move a lot, and pregnant women notice the movements. An ultrasound scan is often done at 20 or 21 weeks to assess anatomy, and the scan can reveal the sex of the preborn baby for couples who want to know.
By week 24, the preborn baby’s lungs are developed and the skin is wrinkled, translucent, and pink to red “because of visible blood in the capillaries.”
Around week 25, a preborn baby begins to respond to sound and to the mother’s movements, and the following week, the lungs start to produce what’s known as surfactant, “the substance that allows the air sacs in the lungs to inflate — and keeps them from collapsing and sticking together when they deflate.” This substance also helps a baby breathe after birth.
At 26 weeks, unique fingerprints and footprints develop, and a preborn baby may react to light as well as sound at this point in development.
At week 27, a preborn baby begins to gain fat, and its nervous system is maturing. At 28 weeks, “the central nervous system can direct rhythmic breathing movements and control body temperature.”
Babies born during this time can survive outside the womb, especially with help from medical technology.
Third Trimester
During the final trimester, a preborn baby undergoes rapid development, including brain growth, bone strengthening, lung maturation, and physical preparation for birth, culminating in full-term readiness by week 38.
Weeks 29 Until Birth
A preborn baby continues to grow and develop through the final trimester of a woman’s pregnancy.
At 29 weeks, the preborn baby can kick, stretch, and grasp. At 30 weeks, it experiences rapid brain growth, and its eyes can open wide.
During the 31st week, the preborn baby’s bones are soft and fully developed and rhythmic breathing through lungs continues. Additionally, the baby can control its body temperature because the brain is maturing and growing rapidly.
Week 32 brings additional changes. For example, the kidneys are developed fully, and the lanugo starts falling off. The baby may sleep longer in preparation for its upcoming birth.
During week 33, the baby’s bones harden, “With exception of the cranial bones around its brain, which needs to be soft to descend the birth canal,” according to the Cleveland Clinic.
At the 34th week, the baby should be turning to a head-down position in preparation for coming into the world very soon.
At week 35, many changes take place. The lanugo is gone, and a thick coating of vernix caseosa, a white, creamy biofilm, covers the skin. Surfactants are still being created in the lungs, and the muscles are almost developed. The mother may experience fewer movements from her baby because it has grown so much in the past few weeks, and there is not as much space for it to move around.
Week 36 brings birth even closer. The baby now has fully formed arms and legs with nails and also develops hair on its head. The earlobes have soft cartilage, and the blood vessels are fully developed. The bones of the skull remain soft in order to allow an easier passage through the birth canal.
By week 37, the baby’s circulatory system is complete, and the bones and muscles are ready for its appearance outside the mother’s womb.
At week 38, a preborn baby is considered full term. Growth stalls at this point, however, the skin becomes smooth, the vernix is shed, and fat cells under skin get plumper.
The final two weeks prepare the preborn baby to leave the cloak of its mother’s womb. The lungs reach full maturity when it enters the outside world.
Growth and development in utero occur throughout all trimesters of pregnancy, with much happening during the first trimester. The heart, brain, lungs, limbs, and so much more develop, change and grow from conception to birth.
A preborn baby’s heart develops and grows quickly during the first trimester, sparking life and includes a four-chambered human heart before the end of that 12-week period.
A miraculous and inspiring knitting together by our Creator God.
