Within 24 hours after insemination has taken place, a mature seed will begin to divide itself into many cells that will end up forming the different parts of a human body. After implantation, the divided cells form a little ball known as an embryo. The developing baby is an embryo until the eighth week, after which it will change its name to a fetus.
The unborn baby will develop throughout three trimesters, each three months long. This is the best way to monitor pregnancy because of the unique developments that occur every three months. Most of the baby’s physical features develop during the first trimester. Therefore, it is important for moms to eat well and take their prenatal vitamins, especially during the first few months.
By the time mom is getting to the last trimester, most of her baby’s organs and physical features have developed, and most fetuses can survive in the outside world if they happen to be born prematurely. Babies spend most of the last few weeks adding some much-needed meat to stay warm. Apart from the internal organs maturing, there are physical traits that develop during those last few weeks.
Here’s a list of the 20 physical traits that the unborn baby develops last. Babies born before their time in the womb is complete may have some complications simply because they weren't done "cooking" yet.
20 Brain Nerves
The brain is usually one of the first organs to form in utero. According to What to Expect, the brain will divide itself into different sections and form what we refer to as the; forebrain, midbrain, and hindbrain. These parts develop at different times while the baby is still in the womb. However, most of the brain develops during those last weeks. The rapid development of neurons and wiring occur during the last trimester.
The brain also changes from its smooth surface and becomes increasingly grooved and indented as the pregnancy develops. Its size expands drastically during the first trimester to about 3.5 ounces and grows to 10.6 ounces when the baby is about to be born. Brain development does not end at birth; it continues way into childhood.
19 Those Tiny Ears Form At Week Sixteen
An unborn baby’s ears begin to develop from around week six. The ear is not distinguishable from an ultrasound because only the inner ear development will have commenced. At around week nine, small indents will begin to develop at the side of the baby’s neck.
According to Healthline, the outer ear is the last to develop; it begins development at around week 16 and continues throughout the first and second trimester. By week 24, the little one is able to detect some limited amount of noises. Between week 25 and 28, a baby is able to respond to voices and noise. Their heart rate goes up when they hear sounds.
18 Fingerprints Are Distinguishable At Week 18
While most organs begin developing in the first few weeks, the unique swirls and whorls on a baby’s finger begin to appear at around week 18. Prints on the hands and foot are usually one of the last physical traits to develop in a growing fetus.
Fingerprints are usually fully developed by the time the fetus is six months old. Fingerprints are unique to each individual and not even identical twins share the same prints. As Scienceline explains, these ridges and patterns all depend on genetic factors and the unique physical conditions surrounding the baby especially the density of the mother’s amniotic fluid.
17 Nails Are One Of The Last Traits To Develop
When pregnant, moms-to-be usually worry about the development of major organs and may sometimes forget about small parts like nails. Most traits develop by the time a woman is in her last trimester. Fingernails and toenails are one of the last physical traits to develop in a baby.
Their development begins at around the 12-week mark and does not end until a woman gets to the third trimester, at around week 32. At week 34, the nails finally reach the babies fingertips, according to Mayo Clinic. They then start hardening and thickening after this despite appearing soft and flexible at birth.
16 Those Chubby Cheeks
Although a baby’s face is usually fully formed by week 24, it may not resemble a normal human face. At this point, the eyes are still the most prominent feature of the face because the sucking pads, which later give way to the formation of cheeks, develop later on, as stated in MyDr.
Cheeks begin to form late into the third trimester around the same time the baby’s body gains size. Therefore, babies who are born before nine months will have very thin cheeks. Lips and cheeks are very essential as they affect the baby’s ability to latch when born, according to Dysphagiacafe. Eventually, after birth, the sucking pads will shrink and allow cheek muscles to develop.
15 Hair Texture And Color Come In At Week 14
Hair follicles begin to develop during week 14 of pregnancy. An unborn baby’s hair is usually pigment-free and bright white during pregnancy but this hair does not make it to birth, according to TheBump. It grows and sheds off just like an adult’s hair.
During the third trimester, lanugo—the hair that grows all over the baby’s body—sheds off. Then the fine hair the baby is born with starts to grow. This is one of the last traits to develop. The texture and color of the hair change over time after birth. Because of their different genetic makeup, not all babies are born with hair, those who do develop it last.
14 Skin Complexion Continues To Grow And Change
The skin is the body’s largest organ. It begins to develop between weeks five and eight, according to WhatToExpect. By week 16, all layers of the skin's epidermis have usually commenced development. However, the development process does not finish until week 32.
The skin complexion, on the other hand, is one of the last traits to develop in unborn babies. When skin starts developing it is usually opaque. According to What to Expect, this is because the little one has very little cushion, making the skin look transparent. A baby’s skin complexion will begin to show once the flesh develops and this happens during the third trimester when the baby’s hormones signal thicker flesh production.
13 Lungs Take Time To Develop
Most vital organs begin development well into the first trimester; however, the lungs take time to develop. Unborn babies do not breathe through their noses while in utero; they get all the oxygen they require from their mothers through the umbilical cord, according to BabyCenter. This is why lungs are amongst the last organs to develop; their work will commence only after birth.
Lung development begins at around week 26 and is not complete until the third trimester. According to What to Expect, airway passages begin to form tubes that take in and expel air. When they develop enough, the baby will breathe in amniotic fluid to exercise his or her lungs.
12 Teeth Exist Below The Gums
While teeth do not pop out in utero, believe it or not, they already exist beneath the gums by the time the baby is being born. Teeth development like other body organs occurs in stages. The primary teeth are the first to develop then followed by the hard tissue forming the permanent teeth as claimed by parents.
Primary teeth buds begin to develop at about the sixth week of pregnancy. The permanent teeth begin developing in buds high inside the gum at week 25 but the first set of molars begin to calcify at birth according to Dental Health. This shows that the tooth development process in utero ends at birth making it one of the last physical traits to develop.
11 Eyelid Movement
Like any other part of the body, the eyes are also controlled by the brain. During week four of pregnancy, the two optic nerves connecting the eyes to the brain begin to form and other cells around them form the lens of the eye, according to What to Expect.
By week eight it becomes a more complex structure that includes the eyelid. By week 16 the baby is able to sense and respond to light even though their eyelids are still closed. A baby’s eyes are fully formed by week 26 and at this point, the baby can finally open his or her eyelids. When this happens, the baby is able to keep them open and blink.
10 Eyelashes And Eyebrows Are Formed To Protect The Baby
Unborn babies develop eyelashes pretty late in pregnancy because they do not need them until birth. This short curved hair growing on the eyelids begins to form by the 28th week. The sole purpose of eyelashes is to protect the eye from dust and other particles according to MyDr.
However, just like hair, all babies are born with differently shaped lashes. Some will have short and curly ones while others will have long and luscious eyelashes. Some will have full volume lashes while others will require makeup to make theirs appear thick and many. The same case applies to eyebrows, which also develop during this same period. The main function of eyebrows is to prevent sweat or water from trickling into the eye socket.
9 Reproductive Organs Can Be Seen By Week 18
According to BabyCenter, the gender of a baby is determined and sealed the moment conception takes place, and by week 18 doctors are able to tell the gender of a baby through an ultrasound that is if she is positioned well for them to see this.
However, the male and female reproductive systems take times to develop. In boys, the family jewels are the last to develop, they descend into the scrotum at week 28. In girls, her lady parts are still too small by this time and do not provide any coverage. They usually grow closer together in the last weeks of pregnancy according to Parent.
8 Baby's Flesh Accumulates Throughout Pregnancy
During the third trimester, now that most major organs have developed, the baby’s main job is to bulk up, as explained in Parents. A baby’s skin is usually very wrinkly when they are growing in utero, but the wrinkles disappear after they start developing flesh reserves.
The flesh that accumulates those last few weeks is essential as it keeps them insulated when they are born. Ideally, it is not useful while in the uterus because the womb has the perfect temperature to keep the baby warm. However, in the last weeks, the baby will gain most of its body mass, which also serves as its source of energy.
7 Skeletal Muscles
According to Embryology, there are three main different types of muscles; the skeletal, cardiac, and smooth. The skeletal muscles are those that move voluntarily, cardiac muscles are only found in the heart, and the smooth muscles are the involuntary muscles in organs.
All three muscles begin to form soon after conception and continue to develop even in the last trimester. By week 27, the baby is able to move around in the womb. According to Parents, while the smooth and cardiac muscles are used from the time of conception. Babies begin using the skeletal muscle after their startle reflexes disappear which is months after giving birth.
6 Iris Color Is One Of The Last Things To Change
The unborn baby’s eye color is usually one of the last traits to develop. Moms may be curious to know whether their little one will be brown-eyed, green-eyed, or blue-eyed but this will have to wait until week 31, according to Parents. This is because the little one’s eyelids remain closed and filled with pigment until week 28.
The iris color begins to appear around week 31, but the color pigment almost changes after birth, it stabilizes between months six and twelve after birth. Iris color is determined largely by genetics, nothing the mother does or eats in pregnancy can change it, according to AptClub.
5 The Tongue Forms In The Fourth Week
The baby’s tongue usually starts to develop during week four. It is the last part of the mouth to form. During the first two months, neurons will start connecting the brain to the baby’s mouth and taste buds. Taste buds normally start to develop where the tongue will be. According to What to Expect, the tongue begins forming in the first trimester and continues into the third.
By week 18, taste pores will have developed and the baby will be able to taste and swallow the surrounding amniotic fluid. At the same time, the tongue is still developing and can grow up to 73mm-long by the end of the second trimester. The number of tastes buds the baby has will continue to increase throughout the second and third trimester.
4 The Chest Becomes More Pronounced At Week 37
The chest, which covers the heart and lungs, becomes more prominent at 37 weeks, according to Mayo Clinic. This is because both the lungs and heart begin developing early in pregnancy but do not complete until the third trimester.
For girls, bust development begins at around week six. The chest area known as the "mammary ridge" will begin developing by thickening. By month six, the ridge will extend all the way to the groin. Each breast bud forms a column of cells, which become separate sweat glands. The process continues until the last weeks of fetal development. Both genders will have nipples before birth.
3 Sense Of Smell
By the time mom is getting to her third trimester, many developments have already occurred. The unborn baby’s senses are one of the last things to develop because they have to wait for organs to develop first. A baby’s sense of smell starts developing between weeks six and seven, all the way to the third trimester.
According to First Time Pregnancy, the olfactory nerves that are used to recognize smell begin developing at week 13 and by week 19 they have made a connection to the brain. By the early third trimester, the baby’s little nose is normally ready to detect any smell. Their sense of smell ramps up during month eight and nine.
2 Digestive System
Unborn babies get all the nourishment they require from their mothers through the umbilical cord. They do not eat with their mouths but eventually, they will have to start practicing in order to prepare themselves for the outside world, as stated by Parents.
Kidneys, which develop between weeks 20 to month nine, enable a baby to pass out urine. According to What to Expect, babies start urinating inside the amniotic sac at 25 weeks but they do not have bowel movements in the womb, which is a good thing. This is because the digestive system is one of the last things to develop. Most babies have their first bowel immediately after birth.
1 Kneecaps Come In During The Last Few Weeks
Bones do not grow from cells. They usually evolve from cartilage through a process known as ossification, according to DocCheck. When babies are born their skeletons have 300 bones and cartilage, these develop and fuse into 206 bones by the time they are adults.
Kneecaps are one of the last traits to develop in unborn babies. Most of the kneecap cartilage develops during the last weeks. However, the cartilage turns into curved kneecaps after the child is born and will continue to develop as they grow. Essentially, babies are born with soft kneecaps. This is why they are able to crawl so easily because of the softness of the patella.
Sources: clevelandclinic.org, whattoexpect.com, doccheck.com, healthline.com,scienceline.ucsb.edu, mayoclinic.org, mydr.com, dysphagiacafe.com, thebump.com,parents.com, dentalealth.ie, aptclub.co.uk, firsttimepregnancy.org, embryology.med.