Week 1 Days

Pregnancy Miracle

The Pregnancy Miracle

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I. OVERVIEW. Figure 2-1 summarizes the events that occur during week 1, following fertilization.

II. FERTILIZATION occurs in the ampulla of the uterine tube.

A. The sperm binds to the zona pellucida of the secondary oocyte and triggers the aero-some reaction, causing the release of acrosomal enzymes (e.g., acrosin).

B. Aided by the acrosomal enzymes, the sperm penetrates the zona pellucida. Penetration of the zona pellucida elicits the cortical reaction, rendering the secondary oocyte impermeable to other sperm.

C. The sperm and secondary oocyte cell membranes fuse, and the contents of the sperm enter the cytoplasm of the oocyte.

1. The male genetic material forms the male pronucleus.

2. The tail and mitochondria of the sperm degenerate. Therefore, all mitochondria within the zygote are of maternal origin (i.e., all mitochondrial DNA is of maternal origin).

D. The secondary oocyte completes meiosis II, thus forming a mature ovum. The nucleus of the ovum is the female pronucleus.

E. The male and female pronuclei fuse to form a zygote.

III. CLEAVAGE is a series of mitotic divisions of the zygote.

A. The zygote cytoplasm is successively cleaved to form a blastula, which consists of increasingly smaller blastomeres (e.g., the first blastomere stage consists of two cells; the next, four cells; the next, eight cells).

B. At the 32-cell stage, the blastomeres form a morula, which consists of an inner cell mass and outer cell mass.

C. Blastomeres are considered totipotent up to the eight-cell stage (i.e., each blastomere can form a complete embryo by itself, which is important when considering monozygotic twinning).

*The age of the developing conceptus can measured either from the estimated day of fertilization (fertilization age) or from the day of the last normal menstrual period (LNMP). In this book, ages are presented as fertilization age.

2-cell 4-cell biastula blastula




Secondary oocyte arrested in metaphase


Blastomere Blastula Blastocyst


Embryoblast Blastocyst cavity

Figure 2-1. (A) The stages of human development during week 1. (B) A blastocyst on day 7.



Secondary oocyte arrested in metaphase

2-cell 4-cell biastula blastula




Embryoblast Blastocyst cavity

Figure 2-1. (A) The stages of human development during week 1. (B) A blastocyst on day 7.

IV. BLASTOCYST FORMATION occurs when fluid secreted within the morula forms the blastocyst cavity.

A- The inner cell mass, which becomes the embryo, is now called the embryoblast.

B. The outer cell mass, which becomes part of the placenta, is now called the tro-phoblast.


A. The zona pellucida must degenerate for implantation to occur.

B. The blastocyst implants within the posterior superior wall of the uterus. During the secretory phase of the menstrual cycle, the blastocyst implants within the functional layer of the endometrium

C. The trophoblast differentiates into the cytotrophoblast and syncytiotrophoblast.


A. Ectopic tubal pregnancy

1. Tliis type of pregnancy occurs when the blastocyst implants within the uterine tube owing to delayed transport.

2. The ampulla of uterine tube is the most common site of an ectopic pregnancy. The rectouterine pouch (pouch of Douglas) is a common site for an ectopic abdominal pregnancy.

3- Ectopic pregnancy is most commonly seen in women with endometriosis or pelvic inflammatory disease.

Conjoined Twins Diagram
Twin Pregnancy DiagramAbdominal Tubal Pregnancy Diagram
Figure 2-2. Diagram of twinning. (A) Dizygotic twins. (B) Monozygotic twins. (C) Conjoined twins.

4. Uterine tube rupture and hemorrhage may occur if surgical intervention (i.e., salpingectomy) is not performed.

5- An ectopic tubal pregnancy presents with abnormal uterine bleeding and uni-lateral pelvic pain, which must be differentially diagnosed from appendicitis, an aborting intrauterine pregnancy, or a bleeding corpus luteum of a normal in-trauterine pregnancy.

B. Twinning (Figure 2-2)

1. Dizygotic (fraternal) twins result from the fertilization of two different secondary oocytes by two different sperm. The resultant two zygotes form two blastocysts, each of which implants separately into the endometrium of the uterus. Thus, these twins are no more genetically alike than are siblings bom at different times.

2. Monozygotic (identical) result from the fertilization of one secondary oocyte by one sperm. The resultant zygote forms a blastocyst in which the inner cell mass (embryoblast) splits into two. Therefore, the twins are genetically identical.

3. Conjoined (Siamese) twins. In these monozygotic twins, the inner cell mass (embryoblast) does not completely split. The two embryos are joined by a tissue bridge (e.g., at the head, thorax, or pelvis).

C. In vitro fertilization (Figure 2-3) requires the sequential application of several of the following techniques.

Conjoined Twins Diagram

Figure 2-3. Diagram of various steps involved with in vitro fertilization.

(3) In vitro fertilization

(2) Sperm collection

Figure 2-3. Diagram of various steps involved with in vitro fertilization.

1. Clomiphene citrate is administered to stimulate multiple ovulation.

2. Oocytes are collected by needle aspiration from the ovary with the assistance of ultrasound visualization.

3. Sperm are collected via masturbation; the sperm are separated from seminal fluid and undergo capacitation by exposure to ionic solutions. In cases of oligospermia (infertility due to a low number of sperm), multiple samples may be obtained over an extended period of time.

4. Sperm and oocytes are cultured together. The success of in vitro fertilization is judged by the presence of two pronuclei with the oocyte.

5. Cleavage is allowed to proceed in vitro to the eight cell stage embryo.

6- Typically, at least three embryos are transferred to the uterus, because there is a low success rate of implantation.

7. The remaining embryos are frozen for future use in case the first embryo transfer does not result in a pregnancy.

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  • angelina genovesi
    How are conjoined twins formed?
    4 years ago

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