The process of giving birth is undoubtedly the most revered thing a living organism can do. While reproduction is the biological purpose of life (to maintain the continuity of the species), people remain in owe of it. A human baby takes about 36 weeks to fully develop in the uterus of a woman – a period known as GESTATION. During these 9 months, rapid changes occur in the fetus growing IN UTERO (in the uterus) and the pregnant woman.
It is no less than a marvel how a baby develops – from a single-celled organism, incapable of performing any function on its own to a well-developed baby who will grow to be an independent person. This process follows a series of well-defined steps that ensure the proper development of the baby. However, a small error in this process can initiate a chain of biological and emotional turmoil on the woman and her family.
How does ‘life’ begin?
The initiation of a new life occurs as a single sperm fertilized an ovum in the FALLOPIAN TUBE of a human female. As the fertilization is successful, the ovum builds a barrier around its boundaries to inhibit the entry of another sperm. At this point, the female and male chromosomes combine, forming the GENOME of the resultant ZYGOTE – which regulates and supervises the growth of the zygote through every stage.
EMBRYOGENESIS is the development of an embryo from a fertilized egg and into a full-fledged organism. Once a fertilized egg attaches to the nutritious UTERINE WALL, the association is called IMPLANTATION. Following implantation, the embryo launches its development into the complex structure of a human.
What is the timeline of the development of a baby?
4 Weeks
The resultant embryo has been receiving nutrients from the uterine wall necessary for its development. There is an intimate connection between the embryo and the uterine wall via a tissue called PLACENTA, present in the UMBILICAL CORD.
While precise organs will not have developed at this point, the embryo conforms to the shape of a baby – which exhibits structures that will soon develop into a face and a neck. ORGANOGENESIS is the process of formation of organs from the GERMINAL LAYERS – namely ectoderm, mesoderm, and endoderm. Organogenesis is initiated at this stage as the heart, liver, lungs, and stomach start to develop. The primitive heart will start beating as the newly formed red blood cells will begin circulation.
8 Weeks
The embryo’s facial features start to develop – such as eyes and ears. Precursors to limbs start to grow, along with structures resembling fingers and toes. The growth of the organ systems is in their conception; the digestive tract, the neural system is in their primitive stages. Since the neural system begins to develop, the FETUS may be able to sense things since the neurons form. From here on out, the embryo is called a fetus.
12 Weeks
The fetus should be capable of movement by now, owing to the now-functional limbs with fingers and toes. As the first trimester comes to a close, mostly all the organ systems in the fetus must have matured. Around this time, the heartbeats of the fetus may be distinct enough to be heard by a physician as the circulatory system begins to function. The gums in the fetus’ mouth would have initiated the formation of teeth. This development is often referred to as CRITICAL DEVELOPMENT.
16 Weeks
The facial features of the fetus have been enhanced with the development of the eyelashes, eyebrows, and eyelids. Fingernails and hair are also formed. Due to the structural stability of the facial features and the limbs, the fetus can move on its own to yawn, blink or moves its facial muscles. This is also the point where the sex of the fetus can be identified since its reproductive organs have developed. This is done by a doctor using a technique called ULTRASOUND.
20 Weeks
Owing to the development of muscles in the limbs of the fetus, the mother may get a sensation of the baby moving, called QUICKENING. This is the stage at which sensitive and soft hair grows on the fetus’ arms, head and back, which clubbed with a layer on the skin of the fetus act as a protective sheath. This layer is called VERNIX CASEOSA. The hair and the vernix caseosa exist to protect the baby from the AMNIOTIC FLUID – this fluid provides a cushioning effect for the fetus in the amniotic sac and imparts it with nutrients required for its growth. This hair and the layer are shed within a week of birth.
24 Weeks
The skin fully develops while the veins may be visible – so do the fingerprints. The fetus can open its eyes due to the proper formation of the eyelids. The baby might start responding to sounds, which is why it is recommended to talk to babies after this time. Since the development of the baby is its peak, a baby born at the end of the second trimester – although premature – can be placed in an incubator and survive.
28 Weeks
The fetus will start storing fat for back-up energy. This energy reserve is necessary for its survival of it is born prematurely. The baby tends to change its position in response to sounds or light – enabling the woman to feel this movement. Since the time for delivery is coming closer, the amniotic fluid starts to diminish.
32 Weeks
The fat deposits under the skin of the baby may cause soft wrinkles on the body – referred to as BABY FAT. The brain of the baby develops rapidly at this stage and might be more in control of its movements and response to stimuli. Some babies may still have under-developed lungs, but all the other organ systems have been perfected and fully-functioning.
36 Weeks
Since this marks the ninth month of pregnancy, the baby is all ready to be delivered; this process is called PARTURITION. The baby in the womb behaves like how a newborn would – in control of its limbs and movements. The lungs of the baby must have developed by now.
How is the baby born?
LABOUR, which is the forceful muscular contractions of the uterine wall during parturition is triggered by the fetus. The fetus signals that it is mature by secreting the hormone OXYTOCIN which diffuses through the placenta and stimulates the contractions. Other hormonal changes, especially the peak of ESTROGEN, causes dilation of the CERVIX. The growing intensity of the contraction causes the woman to push out the baby along with the placenta. The baby is born upside down (headfirst).
Does the development of a baby in utero show homology?
HOMOLOGY is the study of structural similarities between organisms and linking them to a common evolutionary ancestor. Ernst Haeckel, a biologist, had extended upon a theory known as the RECAPITULATION THEORY. This theory correctly exhibits that the development of a fetus in utero follows an evolutionary development pattern. ONTOGENY, the development of a fetus, resembles PHYLOGENY – the evolutionary development of its ancestors. This paved a way for a famous saying in the scientific community – ontogeny recapitulates phylogeny.
Haeckel believed that a human fetus shows stages that closely resembles those of fish, amphibian, avian and mammalian embryos. This can be observed in the image below, procured from Haeckel’s notebook – which compares the development of a human fetus as compared to fetuses of fish, amphibian, avian and mammalian. The testament to the reliability of this theory is that the human fetus is observed to possess gills and tails at some point in their development, much like that of fish and calf.
