Introduction

In this chapter we will discuss the topics covering pregnancy, from conception to birth. The chapter will cover fertilization, implantation of the zygote, to becoming a fetus, the three trimesters, and the progressive development of the fetus through the weeks of pregnancy. It will cover the topic of birth and different birthing methods.

Fertilization is the joining of a sperm and an egg. A sperm is a male gamete that is released into the vagina of a female during intercourse. In order for fertilization to occur there must be a mature ovum present. Every month one of the ovaries releases an egg which will meet one of the approximately  4 million sperm the male ejaculates into the vagina. The sperm swim through the cervix and into the uterus which lead to the fallopian tubes. This is where fertilization is most likely to take place. The high amount of sperm in the ejaculate is needed because only around 100 survive to enter reach the fertilization site. In order to penetrate the egg the sperm must first break through 2 barriers surrounding the ovum. The acrosome of sperm comes in contact with the corona radiata and releases digestive enzymes that break down a gelatinous layer around the egg called, the zona pellucida. Once a sperm reaches the plasma membrane of the egg it sets off a reaction that spreads across the membrane of the egg preventing other sperm from breaking through the egg membrane. Once the sperm reaches the inside of the egg it sheds its tail and the two nuclei fuse and now the 23 chromosomes from the egg and the 23 chromosomes of the sperm join and they become a zygote. Chromosomes contain all the information needed to determine the genetic structure of the new baby. Normally all human beings have two chromosomes that determine sex: A combination of X and Y makes a male or a combination of X and X makes a female. All ovum have X sex chromosomes where as sperm have both X or Y sex chromosomes. Therefore, the male gametes determine the sex of the baby.

An 8-cell embryo in the process of cleavage.

Pre-embryonic Period
After fertilization, the zygote begins a process of dividing by mitosis in a process called cleavage. It divides until it reaches 16 cells. It is now referred to as a morula. As the morula floats freely within the uterus, it starts to bring nutrients into the cells. The morula fills with fluid and the cells inside start to form two separate groups. At this stage it is now a blastocyst. The inner layer of cells is called the embryoblast, and will become the fetus.

The outer layer is called a trophoblast which will develop into part of the placenta. At this
point the zona pellucida is disintegrating. The trophoblast contains specialized cells that
become extensions, like fingers, that grow into the endometrium once in contact with the
well thickened endometrium.
 Implantation
The blastocyst preserves itself by secreting a hormone that indirectly stops menstruation.
The trophoblast cells secrete hCG hormones that help maintain the corpus luteum that
would normally regress. In turn, the corpus luteum continues to secrete progesterone, which
maintains the endometrium of the uterus in the secretory phase. This helps the blastocyst
to continue to grow and stay embedded within the endometrium. The fetal life support
system and the placenta begin to form, and eventually the placenta will take over the job of
producing progesterone.
• Gastrulation and Formation
The embryoblast within the blastocyst forms 3 primary germs layers: ectoderm, mesoderm,
and endoderm.
Ectoderm
This forms the nervous tissue and the epithelium covering the outer body surface. Epidermis
of skin, including hair and nails, glands of skin, linings of oral cavity, nasal cavity, anal
canal, vagina, brain, spinal cord, sensory organs, lens of eye and epithelium of conjunctiva (a
membrane that covers the sclera and lines the inside of the eyelids), pituitary gland, adrenal
medulla, and enamel of teeth.
Mesoderm
This forms all of the muscle tissue and the connective tissue of the body, as well as the
kidneys and the epithelium of the serous membranes and blood vessels. All muscle tissue
(skeletal, smooth, cardiac), all connective tissue (fibrous connective tissue, bone, blood,
cartilage), dentin of teeth, adrenal cortex, kidneys and ureters, internal reproductive viscera,
epithelium lining vessels, joint cavities, and the serous body cavities.
Endoderm
Forms the lining epithelium and glands of the visceral body systems. Lining epithelium and
glands of digestive, respiratory, and parts of urogenital systems, thyroid and parathyroid
glands, and thymus.

Source: Cray MI,  Ch. 15 Pregnancy and Birth,Textbook of Human Physiology and Biophysics, V#1. Atlanta Ga: IVMS 2014:413-15