Definitions:
• Gestational age refers to the duration of pregnancy dated from the first day of the last menstrual period (LMP), which precedes ovulation and fertilization by around 2 weeks.
• From fertilization to 10 weeks’ gestation (8 weeks post-conception), the conceptus is called an embryo. From 10 weeks to birth, it is a fetus. Follicular development and ovulation
• Primitive germ cells are present in the female embryo by the end of the third week of intrauterine life. The number of germ cells in the fetal ovary peak at around 7 million at 5 months. Degeneration occurs thereafter, with only 2 million primary oocytes surviving in the ovary at birth and as few as 300,000–400,000 in the ovary of prepubertal women.
• Primary oocytes have a diploid number of chromosomes (46, XX) which are suspended in prophase of meiosis I. During the follicular phase of the menstrual cycle, several primary oocytes mature under the influence of follicle-stimulating hormone (FSH), with completion of meiosis I. This results in the formation of the secondary oocyte with a haploid number of chromosomes (23, X) and extrusion of the first polar body. The mature follicle is known as a Graafian follicle. Secondary oocytes enter meiosis II but become suspended in metaphase. The selection of a single dominant follicle occurs at this time.
• The midcycle surge of luteinizing hormone (LH) results in ovulation and extrusion of the secondary oocyte into the abdominal cavity.
Fertilization
• Fertilization of a mature ovum by a single spermatozoon (23, X or 23, Y) occurs in the fallopian tube within the first few hours after ovulation. The genetic composition of the spermatozoon thus determines the gender of the conceptus.
• Fertilization serves as a trigger for the secondary oocyte to complete meiosis II. The male and female pronuclei (each haploid) fuse to form the zygote, which has a diploid number of chromosomes (46, XX or 46, XY). Preimplantation embryo development
• Mitotic division of the zygote (known as segmentation or cleavage) gives rise to daughter cells called blastomeres. The initial division results in a two-cell stage followed by a four-cell stage and an eight-cell stage. Such divisions continue while the embryo is still in the fallopian tube. As the blastomeres continue to divide, a solid ball of cells is produced known as the morula.
• The morula enters the uterine cavity around 3–4 days after fertilization. The accumulation of fluid between blastomeres results in the formation of a fluid-filled cavity, converting the morula to a blastocyst.
• A compact mass of cells (the inner cell mass) collects at one pole of the blastocyst. These cells are destined to produce the embryo. The outer rim of trophectoderm cells is destined to become the trophoblast (placenta).
Implantation
• Implantation usually occurs in the upper part of the uterus and more often on the posterior uterine wall.
• Before implantation, the collection of cells surrounding the blastocyst (known as the zona pellucida) disappears and the blastocyst adheres to the endometrium. This is known as apposition.
• The blastocyst then proceeds to invade the endometrium. Implantation is usually completed by day 24–25 of gestation (day 10–11 post-conception).
Early Embryo and Fetal Development
Embryonic development after implantation
• By day 24–26 of gestation, the embryonic disc is bilaminar, consisting of embryonic ectoderm and endoderm.
• Cellular proliferation in the embryonic disc results in midline thickening known as the primitive streak. Cells then spread out laterally from the primitive streak between the endoderm and ectoderm to form the mesoderm. This results in a trilaminar embryonic disk.
• These three germ layers give rise to all the organs of the embryo. The nervous system and epidermis along with its derivatives (lens of the eye, hair) are derived from ectoderm. The gastrointestinal tract and derivatives (pancreas, liver, thyroid) arise from endoderm. The skeleton, dermis, muscles, and vascular and urogenital systems are derived from mesoderm.