FETAL DEVELOPMENT

updated on 17/july/2021

By:Abubakar usman

The awareness that something small and fragile is growing inside of a woman is both wonderful and scary. Wonderful, because a new life would be wriggling inside her very own body. Scary, because nobody could accurately predict the outcome despite everything that technology could handle. However, the feeling of becoming a mother is yet to become the most challenging one a woman could ever have.

Fertilization

Fertilization is the process wherein the ovum and the spermatozoa unite at the ampullary portion of the fallopian tube, the usual site of fertilization.

A mature ovum can only be fertilized within 24 to 48 hours after being released.
The functional life of the spermatozoa is only 48 to 72 hours.
The best time that fertilization would occur is 72 hours after sexual intercourse.
The fertilized ovum is propelled along the tube through the help of the peristaltic movements of the fallopian tube and the tube’s cilia.
The average time that the sperm can reach the cervix is within 90 seconds, and it can reach the outer fallopian tube within 5 minutes.
If the ovum has already been penetrated by a spermatozoon, it changes its composition so that it becomes impermeable to other spermatozoa.
After penetration, the chromosomal materials of both the ovum and the spermatozoon combine to form a zygote.
Three factors determine the certainty of fertilization:
- the maturation of both the sperm and the ovum;
- the ability of the sperm to reach the ovum; and
- the ability of the sperm to penetrate the cell membrane and achieve fertilization.

Implantation

The migration of the zygote towards the uterus reaches 3 to 4 days, and it is propelled by the muscular contractions in the fallopian tube.
Mitotic cell division or cleaving starts after 24 hours. The rate of cleaving is an average of one every 22 hours.
As the zygote reaches the uterus, it already has 16 to 50 cell divisions, and it is now called a morula because of its bumpy appearance.
The morula floats freely along the body of the uterus within 3 to 4 days, and it becomes a blastocyst.
The blastocyst attaches into the endometrium, and this process is called implantation, which occurs 8 to 10 days after fertilization.
Apposition, or the brushing of the blastocyst against the endometrium, is the first part of implantation.
Adhesion occurs afterwards as the blastocyst attaches to the surface of the endometrium, then invasion, as it settles into the folds of the endometrium.
On the day of implantation, the woman may experience a small amount of vaginal spotting as the capillaries by the implanting blastocyst.
As implantation occurs, the zygote now becomes an embryo.

Fetal Structures

Deciduas

The uterine endometrium continues to thicken because of the corpus luteum that is influenced by hCG, and instead of sloughing off in a usual menstrual cycle, it becomes the deciduas.
The deciduas are divided into three parts: basalis, capsularis, and vera.
The decidua basalis is the innermost portion of the layer which rests directly under the embryo.
The decidua capsularis encapsulates the trophoblast’s surface.
The decidua vera becomes the remaining portion of the uterine lining,and sheds as the lochias.
Eventually, the deciduas vera and capsularis fuse because of the enlarging embryo.

On the 11th or 12th day, the chorionic villi start to form from the miniature villi that protrude from a single layer of cells to start the formation of placenta.
The chorionic villi have a central core and fetal capillaries, and a double layer of trophoblast cells.
The syncytial layer or the outer portion of the two layers produces placental hormones such as hPL, hCG, estrogen, and progesterone.
The Langhans’ layer is the middle layer and it protects the embryo and fetus from infectious diseases. This layer appears to function as early as 12 days’ gestation.
The layer disappears on the 20th to 24th week of gestation, however, leaving the fetus more susceptible to infections.

Placenta

The placenta, which is a Latin term for “pancake” for its appearance came from the trophoblast tissues and has a lot of functions that benefit the fetus.

Nutrients such as glucose, amino acids, vitamins, minerals, fatty acids, and water as well as oxygen are transported through the placenta from the maternal blood supply to the fetus.
Placental osmosis also plays an essential part in maintaining the health of the fetus. it is impermeable to a few harmful substances, thereby it does not allow the crossing of these substances towards the fetal blood circulation.
The syncytial layer produces various hormones that benefit both the mother and the fetus.
The human chorionic gonadotropin is the first placental hormone to be produced, and it ensures that the corpus luteum would continue to produce estrogen and progesterone to support the pregnancy.
Estrogen is also one of the hormones produced by the syncytial cells and it aids in the uterine growth and the development of the mammary glands in preparation for lactation.
Progesterone is responsible for maintaining the lining of the uterus during pregnancy. It also reduces the contractility of the uterus to prevent preterm labor.
Human placental lactogen promotes lactogenic properties and mammary growth in preparation for the lactation of the mother.

Amniotic Membranes

amniotic membrane

The smooth portion of the chorionic villi eventually becomes the chorionic membrane which forms the sac that contains the amniotic fluid.
The amniotic membrane forms under the chorion, giving an appearance that seem like they are only one membrane.
The amniotic membrane is also responsible for producing the amniotic fluid and the phospholipids that triggers the formation of prostaglandins, the hormone that initiates uterine contractions.

Amniotic Fluid

The normal amount of amniotic fluid is 800 to 1000 mL.
The role of the amniotic fluid in the safety of the fetus is it protects the fetus from trauma or pressure to the mother’s abdomen. It also regulates the temperature for the fetus and aids in muscular development allowing the fetus to move freely
The amniotic fluid also protects the umbilical cord from trauma and pressure, thereby protecting the fetal oxygen supply.

Umbilical Cord

The amnion and chorion compose the umbilical cord which connects the embryo to the chorionic villi of the placenta.
The main function of the umbilical cord is the transport of oxygen and nutrients from the placenta to the fetus and the return of waste products from the fetus to the placenta.
The cord is made up of a gelatinous mucopolysaccharide called Wharton’s jelly that protects the vein and arteries from trauma.
The umbilical cord contains only one vein, which carries blood from the placenta to the fetus, and two arteries, which carries blood from the fetus to the placenta.

Fetal Milestones

4th Week of Gestation

Spinal cord is formed and fused at the midpoint.
Head folds forward and is prominent.
The back is bent, which makes the head almost touch the tail.
A prominent bulge appears which would later form as the heart.
Lateral wings, the body, folds forward and fuse at midline.
Arms and legs are budlike structures.
Eyes, ears, and nose are barely recognizable.

8th Week of Gestation

Organogenesis is achieved and complete.
The heart already developed a septum and valves and is beating rhythmically.
Arms and legs have developed.
Facial features are noticeable.
The genital starts to form but is not yet recognizable.
Fetal intestine is rapidly growing.
Results of an ultrasound would show a gestational sac which confirms pregnancy.

12th Week of Gestation

The toes and fingers already have nail beds.
Faint fetal movements are starting.
Early reflexes are present.
Tooth buds are forming.
Formation of bone ossification centers initiate.
The genital is already recognizable through its appearance.
Urine secretion begins but is not yet evident.
Heartbeat could be detected by Doppler.

16th Week of Gestation

An ordinary stethoscope could detect the fetus’ heart beat.
Lanugo has started to form.
The pancreas and liver are forming.
Urine is present in the amniotic fluid.
Fetus starts to swallow the amniotic fluid.
Ultrasound could determine the sex of the fetus.

20th Week of Gestation

Mother could sense spontaneous fetal movements.
There is hair formation on the head until the eyebrows.
The upper intestine contains meconium.
Brown fat starts to form behind the kidneys, sternum, and posterior neck.
Vernix caseosa also starts to form and covers the skin.
Passive antibody transfer begins.
The sleep and activity patterns of the fetus are evident.

24th Week of Gestation

Lung surfactant begins to develop.
Meconium is present at the rectum.
Eyebrows and eyelashes are distinguishable.
Eyelids can now open.
Pupils react to light.
The fetus has reached the age of viability, wherein they could survive externally if cared for in a modern intensive facility.
Responds to sudden sounds.

28th Week of Gestation

Surfactant is demonstrated in the amniotic fluid.
Alveoli are starting to mature.
Testes descend into the scrotal sac.
Retinal blood vessels start to form but are highly susceptible to damage.

32nd Week of Gestation

Subcutaneous fat is deposited.
Fetus responds to sounds outside the mother’s body through movements.
Active Moro reflex is present.
Iron stores are starting to develop.
Fingernails are starting to grow.

36th Week of Gestation

Depositions of iron, carbohydrate, calcium, and glycogen stores are in the body.
Additional subcutaneous fats are deposited.
One or two creases are present at the sole of the foot.
Lanugo starts to diminish.
Some babies turn and assume a vertex presentation.

40th Week of Gestation

Fetus now kicks very actively and hard enough to cause discomfort.
The fetal hemoglobin is being converted to adult hemoglobin.
Vernix caseosa is fully formed.
Fingernails extend to the fingertips.
The soles of the feet have creases that cover at least two-thirds of the surface.

The slow but sure development of the fetus inside a woman’s body should be monitored to ensure the delivery of a healthy and safe baby. Fetal development is a critical stage in a mother’s responsibility over her children. The role of a mother starts not only during the time that the baby is born, but most especially when she decides that she wants to conceive an offspring.