Pregnancy Stages, Development & Changes: A Guide
Fertilization
The process by which two gametes fuse to create a new individual with a genome derived from both parents.
It is caused by the entry of the sperm inside the egg, which then forms a coating around it that prevents fertilization by a second sperm.
Nesting
The fertilized egg travels to the uterus and implants in the uterine membrane, which has been previously prepared by hormone action to welcome, shelter, and nourish it. At this time, the embryo consists of two distinct classes of cells:
- One that focuses primarily on the formation and development of the embryo.
- One that aims to establish a link with the uterus for embryo nutrition.
Implantation
Implantation is an event that happens in the first days of pregnancy in which the embryo attaches to the uterine wall. From this point, prenatal development begins. This implantation allows the fetus to receive oxygen and nutrients from the mother for its growth and development.
Process of Embryonic Development
Embryonic development is the study of the developing organism from the zygote or fertilized egg. The events in embryonic development are under the control of hereditary factors, the genes that direct and control the steps in the transformation of a single fertilized egg to a fully formed embryo.
Basic Process and Early Embryonic Development
Embryonic development is considered to begin when fertilization is effected. Among the first visible events that follow fertilization is a series of mitotic cell divisions called cleavage. The fertilized egg divides into two cells, then 4, 8, and so on.
The segmentation usually forms a hollow sphere called a blastula, consisting of a layer of hundreds of small cells. The state of segmentation ends with the formation of the blastula.
Amniotic Fluid
Amniotic fluid is a liquid produced by the fetus and surrounding it during pregnancy. It acts mainly as protection for the fetus and provides a stable temperature, a buffer against external trauma, and allows freedom of movement necessary for normal lung development.
It is a liquid that surrounds and cushions the embryo and then the developing fetus inside the amniotic sac.
Yolk Sac
The yolk sac membrane is an annex attached to the embryo that provides nutrients and oxygen to the embryo in fish, sharks, reptiles, birds, and primitive mammals, as well as eliminates metabolic waste. In the human embryo, it acts as the circulatory system in the early stages of development, before the start of internal circulation. The yolk sac is covered by a thin mucous layer and an inner wall rich in blood vessels that will provide nutrients to the embryo. After gastrulation, the yolk sac is suspended and supports the embryonic body, forming the vitelline duct.
Cord
It consists of a monolayer of flat cells connected together. It faces the space of the yolk sac. The cells of this layer have a large number of microvilli. From the seventh week, there are degenerative changes in the epithelium, forming deep surface roughness and increasing the amount of mucus, causing the collapse of microvilli. They undergo a process of fragmentation and detach from the cell surface. The remaining cytoplasmic organelles involute, leaving a very vacuolated cytoplasm.
Placenta
The placenta is an ephemeral organ present in most mammals (except marsupials, for example). It is produced by the mother and relates closely to the baby, meeting the needs of breathing, nutrition, and excretion of the fetus during its development. The placenta develops from the same cells from the sperm and egg that gave rise to the fetus and has two components: a fetal portion (the smooth chorion) and a maternal portion (the chorionic decidua).
Fetal Circulation
Fetal and neonatal circulation have particular characteristics. Knowledge of these characteristics allows us to understand why some heart diseases (e.g., severe eutrophic born, where the two ventricles handle equal pressures) worsen a few days after birth or why heart failure occurs around the first month of life in heart diseases with increased pulmonary flow.
Pregnancy and Duration
Stages of Pregnancy
Month 1
The fetus measures 4 mm and weighs 1 g. Early development of the head occurs. The heart beats.
Month 2
The fetus measures 3 inches and weighs 3 g. Development of arms and legs, as well as the brain and internal organs, occurs.
Month 3
The fetus measures 10 cm and weighs 45 g. Development of the eyelids and limb movement occurs.
Month 4
The fetus is 15 cm and weighs 180 g. It is covered with lanugo. The intestine begins to fill with meconium. The skin is still very thin, almost transparent.
Month 5
The fetus measures 18 cm and weighs 500 g. The hair of the head, eyelashes, and eyebrows grow. Development of the immune system occurs.
Month 6
The fetus measures 25 cm and weighs 1000 g. The face is already fully formed. The skin is covered with a fatty material called vernix caseosa. The fetus opens its eyes and moves a lot.
Month 7
The fetus measures 30 cm and weighs 1500 g. The lungs begin to move. Subcutaneous fat increases, and the fetus does not fit well in the womb.
Month 8
The fetus is 35 cm and weighs 2500 g. It generally turns upside down (vertex). The skin thickens, acquiring the pinkish color it will have at birth.
Month 9
The fetus measures 50 cm and weighs 3000 g. The lungs are now completely developed for life outside the womb. Lanugo falls out, and the skin stretches.
Signs and Symptoms of Pregnancy
Signs of Presumption
- Interruption of menstruation is the most common symptom indicating that a woman may be pregnant.
- Nausea and vomiting often occur in the morning, caused by the change in carbohydrate metabolism and the increase in hormones.
- Increased frequency of urination occurs because the enlarged uterus presses on the bladder, thus decreasing its functional capacity.
- Hypersensitivity and breast augmentation can occur in the first weeks of pregnancy.
- Perception of the first fetal movements: These are first felt in the second half of the fourth month.
Signs of Probability
- Abdominal changes: The abdomen becomes more voluminous due to the enlarged uterus.
- Hegar’s sign: This sign refers to the softening of the tissue between the neck and body of the uterus.
- Chadwick’s sign: The doctor may note a purple vaginal mucosa due to increased blood supply.
- Ballottement
- Lab Test: Determines the quality and quantification of human chorionic gonadotropin.
Positive Signs
- Spontaneous fetal movement: Between the fourth and fifth month.
- Noise:
- Fetal heartbeat
- Funicular sounds
- Uterine sounds
- Observing the skeletal outline
Gravity Changes
- During pregnancy, many changes occur in the mother’s body as it adapts to the needs of the fetus.
- The heart and respiratory rate increase to send oxygen and nourishment to the fetus through the placenta and eliminate waste.
Physiological and Metabolic Changes of Pregnancy
In Breast
- By the sixth week of pregnancy, and in response to elevated levels of estrogen and progesterone, a feeling of fullness, increased sensitivity, itching, and heaviness of the breasts begins.
- The nipples and areolas become pigmented, and secondary pink areolas develop, which extend beyond the primary ones.
- Around the nipples, hypertrophy of the sebaceous glands embedded in the primary areola, called Montgomery’s tubercles, can be observed.
- The blood vessels running under the skin dilate, become visible, and appear as a blue complex network beneath the surface of the skin.
- During the second and third trimester, the growth of the mammary gland increases.
- The increase in glandular tissue makes the connective tissue move, and as a result, it becomes softer and looser.
Metabolic Changes of the Vagina
Vaginal flora undergoes changes over the life of the woman due to the hormonal cycle, pregnancy, and menopause. These intimate balance changes are normal but may indirectly favor the colonization of pathogens.
Physiological Changes in the Vulva
- After having a baby, the vulva is spacious with smooth walls that gradually decrease in size.
- Metabolic changes in the vulva do not alter its thickness or color.