MATERNAL PHYSIOLOGIC pdf

77

FERTILIZATION AND IMPLANTATION

THE PLACENTA

The extruded ovum is directed into a uterine tube by its fimbriae

and the peritoneal fluid currents. Normally, a few hours after in￾semination, spermatozoa will have passed through the cervix and

uterus into the fallopian tubes. Capacitation of the sperm (prepara￾tion for fertilization) occurs between its passage into the cervix and

its reaching the midportion to outer portion of the tube. Fertiliza￾tion occurs when a spermatozoon penetrates the ovum, usually in

the outer portion of the fallopian tube. It is unusual for fertilization

to occur more than 24 h after ovulation. Indeed, if this occurs, an

ectopic pregnancy may result.

The fertilized ovum rapidly develops into an embryonic blasto￾cyst. About 3–4 days are required for this minute, free-floating ob￾ject to reach the uterus. Until implantation, the zygote is nourished

by adherent granulosa cells and tubal fluids. Progress is mainly by

tubal ciliary action, but peristalsis probably contributes to tubal tran￾sit. Endometrial implantation ensues 5–6 days after fertilization.

The favored sites are the anterior and the posterior fundus. This is

summarized as follows.

Last menstrual period (LMP) Cycle days 1–7

Ovulation Day 14 after LMP

Fertilization Day 14–15 after LMP

Ovum transit tube to uterus Days 15–19

Ovum free in uterus Days 19–21

Implantation Day 19–21 after LMP

Expected period Missed or scanty

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MATERNAL PHYSIOLOGIC

ADJUSTMENTS TO PREGNANCY

CHAPTER

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BENSON & PERNOLL’S

78 HANDBOOK OF OBSTETRICS AND GYNECOLOGY

The early developing embryo with its tissue layers is shown in

Figure 4-1.

The chorion, or protective covering of the fertilized developing

ovum, has an outer ectodermal layer (trophoblast). The inner layer

is mesenchyme. The trophoblast, initially a poorly defined syn￾cytium, soon develops into two tissue types: an outer confluent but

differentiated plasmotrophoblast (syncytio- or syntrophoblast) and

an inner distinct cytotrophoblast (Langhans’ striae).

The trophoblast produces proteolytic enzymes capable of rapid

destruction of endometrium and even myometrium. This allows

the zygote to erode quickly into the functionalis layer of the

FIGURE 4-1. Relationships of the fetus, placenta, and membranes to the

uterus in early gestation. Top left, 4 weeks. Top right, 6 weeks. Bottom left,

18 weeks. Bottom right, 22 weeks.

(From K.L. Moore, The Developing Human. W.B. Saunders Co., 1973.)