Pregnancy

Fertilization between the egg and sperm occurs within the fallopian tubes. The relatively long distance for sperm to travel to this location, along with the acidic pH of the vagina (~4.0), serve to eliminate potentially poorly developed sperm from the selection process. The pre-embryonic period (1–2 weeks) begins with the rapid division of the 1-cell zygote into a “ball” of 16 embryonic cells known as a morula. Further development from a morula into a blastocyst (200–300 cells) occurs over a period of 5 days or so. By this time, the blastocyst has traveled down the fallopian tubes and into the uterus where it implants into the uterine wall. The outermost layer of cells of the blastocyst forms the trophoblast, which attaches the fertilized ovum to the uterine wall and serves as a nutritive pathway for the embryo; eventually, it will differentiate into the amnion, the placenta, and the umbilical cord. The inner cell mass of the trophoblast will become vascularized to form the chorion; a portion of this becomes the chorionic villi which allows for the exchange of oxygen, carbon dioxide, and nutrients between the mother and fetus. The chorionic villus is what connects the chorion of the fetus with the placenta, which is the bridge structure between the mother and fetus. The placenta begins to form on Day 11 post-fertilization. Take note of the color of the 2 umbilical arteries and 1 umbilical vein. The single umbilical vein is taking oxygenated and nutrient rich blood from the placenta to the fetus, while the double umbilical arteries carry deoxygenated and nutrient depleted blood from the fetus back to the placenta. Remember, that the umbilical vein turns into ligamentum teres in an infant

Pregnancy is divided into approximately three divisions of three-month lengths. The 1st trimester extend from fertilization to up to the first 12 weeks. This is the most vulnerable stage for the embryo. The 2nd trimester is from weeks 13 to 24, this is the period where most organs are completely developed, thus it is possible to see through sonography the anatomical details of the fetus. The 3rd trimester is from weeks 25 to birth, this is where the fetus rapidly grows and its organs are able to support life outside of the womb.

Another way of determining prenatal growth is through these 3 different stages: pre-embryonic (weeks 1-2), embryonic (weeks 3-9), and fetal stages (weeks 9 to birth).

To avoid spontaneous abortion of the newly fertilized egg, a spike in human chorionic gonadotropin (hCG) after implantation occurs within the mother’s blood. The presence of hCG is responsible for maintaining the corpus luteum, which secretes progesterone during the first trimester. Progesterone aids to thicken the uterine lining and promote its formation of new blood vessels to aid the growing embryo. hCG is the main hormone that sustains pregnancy within the first trimester. During this time, progesterone and estrogen begin to rise and eventually take over during the 2nd and 3rd trimesters to maintain the structures required for pregnancy and fetal development.

Pregnancy is a long-term stressful that requires the maternal system to adapt (slowly) to the ongoing changes with fetal development. In fact, there are too many hormonal, physiological, and anatomical changes to be covered here. In short, as the size of the fetus increases, the maternal organs shift upward, which decreases the volume of the stomach and compresses the intestines. The amount of ingested food by the mother per meal decreases and gastric motility slows. There is an approximate 4 cm upward push of the diaphragm which lowers overall functional reserve capacity. There is a ~75% increase in total blood volume, which is attributed to an increase in plasma volume and red blood cell production. This results in an increase in cardiac stroke volume and, therefore, cardiac output. The consequence of these hematological changes results in renal blood flow increasing by ~80% and the volume of the maternal ureters may increase 25× their capacity when compared to non-pregnancy.

Because the fetus is in an aqueous environment, its circulation is slightly different than what is found in the infant. These structures/openings found in the fetus, changes and become fetal remnants in a newborn baby. There are three areas within the fetus’s cardiovascular system that shunt/direct blood in different directions:

1. The ductus venosus allows the blood to (mostly) bypass the liver; this route sends the blood directly to the fetal vena cava. This becomes the ligamentum venosum in an infant.

2. The foramen ovale is a hole in the heart that allows the blood to bypass the right ventricle and, therefore, pulmonary circulation. It becomes the fossa ovalis in an infant. Rememer, that fossa means a depression.

If the foramen ovale does not close shortly after birth, there will be a mixing of deoxygenated and oxygenated blood in the infant. This results in a compromised oxygen delivery to the body that manifests as a blueish color in the infant’s complexion. This is generally known as cyanosis.

3. The ductus arteriosus is a vascular bridge that allows the blood to bypass the pulmonary circuit. It bridges the pulmonary trunk and the aorta. Which becomes the ligamentum arteriosum in an infant