The physiology of conception and pregnancy

Chapter 20 The physiology of conception and pregnancy

Chapter overview

This chapter focuses on the physiological processes associated with conception and pregnancy. This begins with the formation of the gametes in the female and male reproductive systems, and continues with an overview of embryology from fertilisation until birth. It includes the simultaneous development of the placenta to support the embryo, and the changes of pregnancy occurring in the woman. A good understanding of these physiological processes is essential if a midwife is to make informed decisions with regard to monitoring of the woman and fetus and with regard to interpretation of test results. It also enables informed discussion with the woman regarding the changes occurring to her body as the fetus develops, and an understanding of the factors that contribute to a healthy environment for normal embryological and fetal development.

For ease of study, the development of the embryo and placenta and the changes in the woman through pregnancy are presented as separate topics, but the processes are interwoven and simultaneous. The scope of this chapter is limited, but further reading is provided at the end of the chapter for a more in-depth exploration of specific topics. Some of the online resources listed provide animations that can help with the understanding of complex anatomical changes.


The physiological roles of the female and male reproductive systems are to produce and maintain sex cells (gametes), to transport the gametes to the site of fertilisation, and to produce and secrete sex hormones, which control the reproductive process. The female reproductive system has the additional role of supporting the developing fetus. The structures of the female and male reproductive systems underpin these roles.

Anatomy of the female reproductive system

The female reproductive system consists of a pair of female gonads, commonly referred to as the ovaries, as well as a number of accessory ducts, specifically the uterine or fallopian tubes, the uterus and the vagina. The ovaries and the accessory ducts make up the internal female genitalia. The external female genitalia are referred to as the vulva and consist of the mons pubis, labia majora, labia minor, clitoris and vestibule. The mammary glands of the female are also important in the reproductive system following reproduction.

Female internal genitalia

The structures of the female internal genitalia are depicted in Figure 20.1.

The ovaries have two important roles in the reproductive system: to produce female sex cells (oocytes) through the process of oogenesis; and to produce the female sex hormones—the oestrogens (oestradiol, oestrone and oestriol) and progesterone.

The uterine tubes link the ovaries and the uterus, although there is no anatomical connection. The distal ends of the uterine tubes end in fimbriae, which waft the ovulated ovum towards the opening of the tube. Contractions of smooth muscle in the walls of the uterine tubes and the wafting action of cilia lining the walls aid oocyte movement (fertilised or not) to the uterus.

The uterus is a hollow cavity consisting of three regions: the fundus, the body and the cervix. The fundus is the region superior to the uterine tubes. The body is the major portion of the uterus and is the site where implantation occurs. The cervix is the region that connects the uterus with the vagina and consists of the cervical canal, the internal os and external os. The uterus is a thick-walled organ with three layers: the perimetrium (outer layer), the myometrium (thick muscle layer) and the endometrium (the inner mucosal lining). The endometrium consists of two layers: the stratum basalis, which lies next to the myometrium, and the stratum functionalis, the outer mucosa that is shed during menstruation (Fig 20.2).

The vagina is often referred to as the ‘birth canal’, and is the region of the internal female genitalia that connects to the external female genitalia. The vagina is a passage for semen during intercourse and for menstrual blood during menstruation. The epithelial cells that line the vagina produce glycogen, which is metabolised to lactic acid by the vaginal microflora. This results in the vagina having an acidic pH, which protects against foreign microorganisms as well as destroying many sperm cells that enter the vagina.

The female pelvis

The pelvic bones provide points of attachment for the lower limbs and transmit the weight of the trunk to these limbs, and they provide support to the pelvic organs. The size and shape of the female pelvis in relation to the fetal skull is an important determinant of successful childbirth.

Bones of the pelvic girdle

Pelvis, true or false

The pelvis is described in terms of a true and a false pelvis. The true or lesser pelvis is almost entirely surrounded by bone, including the inferior region of the ilium, ischium, pubis and sacrum, and is separated from the false pelvis by the pelvic brim. The false or greater pelvis is superior to the pelvic brim. The false pelvis serves only to support the abdominal viscera and so, unlike the true pelvis, does not have a potentially restrictive role in childbirth (Stables & Rankin 2005). The true pelvis is considered in three planes:

The maximum diameter of the pelvis through which the infant’s head can pass differs for all three planes. Initially the largest diameter is in the transverse plane (pelvic brim), but the anteroposterior diameter becomes the largest at the pelvic outlet, particularly in females who possess a classical gynaecoid pelvis where the angle of the subpubic arch is equal to or greater than 90 degrees. Measurements of the diameter of the pelvis at the brim, cavity and outlet are presented in Table 20.1.

The shape of the pelvis can vary in individuals. While the gynaecoid is most common in women, three other types have been described: the android, anthropoid and platypelloid pelvis. These are illustrated in Figure 20.7.

The shape of the pelvis does not necessarily predict whether a successful vaginal delivery is likely; the size of the fetal head in relation to the pelvis is more important (Stables & Rankin 2005).

Childbirth is dependent on the ability of the baby to move, to ensure that the maximum diameter of the baby’s skull moves to align with the maximum diameter of the pelvis. These manoeuvres are called the mechanism of labour and are outlined in the later chapters on this topic.

Anatomy of the male reproductive system

The structures of the male reproductive system (Fig 20.8) include the testes, penis, accessory ducts (epididymis, vas deferens or ductus deferens, ejaculatory ducts and urethra) and the accessory glands (prostate, seminal vesicles and bulbourethral glands).

The paired testes each contain coiled seminiferous tubules, which are the site of sperm production. The sperm are moved from the seminiferous tubules to the coils of the epididymis by peristalsis. Secretions from the Sertoli cells, adjacent to the seminiferous tubules, build up pressure and contribute to the movement of sperm (Fig 20.9).

In the epididymis, sperm mature and become motile after approximately 14–21 days. During ejaculation, peristaltic contractions move spermatozoa from the epididymis to the vas deferens, through the ejaculatory duct and into the urethra. The urethra runs from the urinary bladder to the outside of the body, with part of the urethra (spongy urethra) running through the penis. The urethra has the dual role of transporting urine and sperm to exit the body.

During sexual excitement, the three columns of erectile tissue surrounding the urethra become engorged with blood, resulting in erection. This enables the penis to enter the vagina during sexual intercourse. Further stimulation leads to ejaculation of semen (sperm and secretions).

Accessory sex glands and semen

Semen is the alkaline fluid that protects, nourishes and transports sperm during ejaculation. Two to five millilitres is ejaculated during orgasm, containing 50–100 million sperm/mL. At least 20 million sperm/mL are necessary for fertility. However, 90% of the fluid is made up of secretions from the seminal vesicles, prostate gland and bulbourethral glands.

Seminal vesicles release alkaline secretions into the ejaculatory duct that contain several chemicals, including fibrinogen, fructose and prostaglandins. Fibrinogen helps to coagulate semen just after ejaculation, and this contributes to retention of semen in the region of the uterine cervix until the sperm become motile. Fructose nourishes the sperm, and prostaglandins aid fertilisation by making the cervical mucus more receptive. Prostaglandins also stimulate uterine contractions, which suck semen into the uterus and fallopian tubes, enabling them to reach the ampulla as soon as five minutes after ejaculation, or up to six hours later. These secretions also dilute epididymal inhibitory factor, which is thought to suppress the motility of spermatozoa.

The prostate gland produces an acidic, milky fluid containing citric acid and clotting enzymes that aid coagulation, and this is emptied into the urethra. Bulbourethral (Cowper’s) glands produce alkaline fluid that neutralises the acidic urethra, along with mucus that lubricates the urethra and protects spermatozoa. Secretions travel through the ducts into the spongy urethra.


Fertilisation involves the union of two gametes—an oocyte from the female and a sperm from the male. Gametes are highly specialised sex cells (Fig 20.10). Gametogenesis (formation of the sex cells) necessarily involves halving the number of chromosomes in each cell, by a type of cell division called meiosis, and by altering the shape of the cells. In females this process is termed oogenesis and in males it is spermatogenesis. When the two gametes join at fertilisation, the full number of chromosomes is restored.


Oogenesis is the process by which mature female sex cells are formed. Much of this process occurs in the fetus prior to birth. Oogonia in the fetus multiply by mitotic division and develop into primary oocytes (containing 46 chromosomes). The first meiotic division also begins before birth, but is arrested until after puberty. Completion of the first meiotic division is linked to the ovarian cycle. Each month, following puberty, several primary oocytes begin to develop but only one matures to complete the first meiotic division 36–48 hours before ovulation (Blackburn 2007). This division results in a large secondary oocyte containing most of the cytoplasm, and a smaller non-functional polar body that soon degenerates. Each has 23 chromosomes (22 autosomes and 1 sex chromosome). At ovulation the secondary oocyte begins the second meiotic division, but the division is only completed if fertilisation occurs. Once again, almost all the cytoplasm goes to one cell, the mature oocyte; a second polar body is non-functional and degenerates. The cellular divisions of oogenesis are outlined in Figure 20.11.

The mature oocyte is a large, immotile cell, just visible to the naked eye. Up to two million primary oocytes are usually present in the ovaries of a newborn female infant, but many regress during childhood, leaving about 40,000 at puberty. During the reproductive life of a female, about 400 oocytes mature and are released during ovulation.

Female reproductive cycles

Females undergo monthly reproductive cycles starting at puberty and continuing through the reproductive years. These cycles are controlled by the hypothalamic–pituitary–ovarian hormones and result in changes in the ovary that lead to release of one secondary oocyte per month, and in changes in the uterus in preparation for implantation of the fertilised ovum. If fertilisation does not occur, the cycle begins again under the influence of the hypothalamic–pituitary hormones. For simplicity, the reproductive cycle is described as an average 28-day cycle. There is considerable individual variation, but the cycle length of most women is between 21 and 35 days (Blackburn 2007).

Ovarian cycle

The ovarian cycle (Fig 20.13) includes three phases: the follicular or pre-ovulatory phase, the ovulatory phase or ovulation, and the post-ovulatory or luteal phase.

Follicular phase

In the follicular phase, the ovarian follicles, containing an oocyte, mature. Ovarian follicles are found in the cortex of the ovary and contain a variety of cells, including primary or secondary oocytes, granulosa cells and theca cells. Oocytes are surrounded by a thick membrane, the zona pelucida, and in mature follicles there is a large fluid-filled cavity, the antrum.

Follicles are present in several forms: primordial (the most undeveloped), primary, secondary and Graafian or vesicular follicles (the most mature).

The completion of meiosis I, in which a secondary oocyte and the first polar body are formed, occurs 36–48 hours before ovulation (Blackburn 2007). Meiosis II continues but is not completed until fertilisation occurs. The process of meiosis results in the genetic material in each oocyte being halved from 46 chromosomes (2n) to 23 chromosomes (n).

Uterine cycle

The uterine cycle occurs in the same timeframe as the ovarian cycle and is similarly controlled by changes in hormonal levels (Fig 20.14). It consists of three stages: the menstrual phase (days 1–5), the proliferative phase (days 6–14) and the secretory phase (days 15–28). The following is a review of each stage.

Hypothalamic–pituitary–ovarian (HPO) axis

The female reproductive system is regulated by a variety of hormones produced by the brain (the hypothalamus and the pituitary gland) and the ovaries (Fig 20.15).

Pituitary gland

The anterior pituitary gland produces and secretes many hormones, including FSH, LH and prolactin. LH and FSH are glycoproteins that enhance each other’s actions (act synergistically) in regulating the ovarian cycle.

FSH, along with oestrogen produced by the ovaries, acts on granulosa cells of the follicles to stimulate growth and produce FSH receptors and LH receptors. The LH receptors combine with LH during the LH surge to inhibit the growth of granulosa cells and initiate production of progesterone from the follicle. The granulosa cells of a primordial follicle undergo limited growth without FSH. Secretion of FSH peaks mid-cycle at a lower level than the LH surge (Blackburn 2007).

LH acts on thecal cells, instigating the production of androgens that are converted to oestrogens by the granulosa cells. This increases the oestrogen level and inhibits the development of other follicles by inhibiting FSH (negative feedback). At the same time, one follicle becomes dominant and secretes high levels of oestradiol, allowing granulosa cell development in that follicle. As levels of oestradiol increase, the release of GnRH is triggered by positive feedback, allowing increased levels of LH and FSH to complete the maturation of follicles, and an LH surge triggers ovulation within 12–24 hours.

Following ovulation, increasing levels of oestrogen and progesterone exert a powerful negative feedback effect, inhibiting the release of GnRH and subsequently LH and FSH. This negative feedback prevents further maturation of follicles—important if the oocyte released at ovulation was successfully fertilised. However, in most cycles, fertilisation does not occur and the corpus luteum degenerates into the corpus albicans, resulting in a decreased level of progesterone. The low progesterone levels cease to inhibit GnRH, which begins to rise, followed by an increase in LH and FSH and the beginning of the follicular phase.

Prolactin is a protein hormone that stimulates milk production. Levels of prolactin rise during pregnancy, but human placental lactogen (hPL) and progesterone prevent the binding of prolactin to receptors in the breast. After delivery, prolactin stimulates milk production.

Research activity

A review publication in the journal Drug Safety by Al-Shawaf et al (2005) suggests that as many as 15% of couples may experience problems with fertility and that pharmacological agents are now routinely used with the aim of retrieving multiple oocytes to increase the chance of pregnancy. They describe the side-effects associated with fertility drugs as changes to the ovarian cycle, ovarian hyperstimulation syndrome and congenital malformations, although they suggest this may relate to the high-risk populations studied rather than the drugs themselves. High-order multiple pregnancies are also identified as being commonly associated with the use of fertility drugs—due to multiple ovulations or more than one embryo replacement. Gonadotrophin-releasing hormone (GnRH) agonists are widely used in assisted reproduction, and while there is considerable research into the possible link of GnRH with increasing risk of ovarian cancer and possibly breast cancer, these authors suggest that this has not been substantiated.


The ovaries produce the oestrogens (oestradiol, oestrone and oestriol), progesterone and inhibin.

Oestrogens are produced in the ovaries by granulosa cells within the follicles and the corpus luteum. The most abundant oestrogen during a woman’s reproductive years is oestradiol, which is responsible for most of the effects of oestrogen in the female reproductive system. However, during pregnancy, oestriol becomes the main oestrogen secreted (Blackburn 2007). The stimulus for oestrogen release is primarily FSH and, to a lesser extent, LH. Oestrogens are responsible for various functions, including:

Progesterone is produced in the ovaries by granulosa cells, primarily of the corpus luteum. Therefore progesterone levels peak in the post-ovulatory phase when the corpus luteum is active, and fall if pregnancy does not occur and the corpus luteum degenerates. LH is the stimulus for progesterone release. Progesterone has various functions, including:

Inhibin is produced in the ovaries by granulosa cells of the follicles and corpus luteum. It exerts negative feedback controls on FSH release during the growth of the dominant follicle and the corpus luteum.

Summary: homeostatic control of the reproductive cycle

While the follicular phase of the cycle is characterised by the positive feedback of oestrogen prompting an LH surge, the luteal phase is characterised by negative feedback. GnRH, LH and FSH production is controlled by negative feedback mechanisms linked to the ovarian hormones in the HPO axis (oestrogens, progesterone and inhibin). If the secondary oocyte is not fertilised, the ovum dies in 24 hours and the corpus luteum deteriorates. The levels of oestrogens and progesterone therefore drop, triggering the production of GnRH, LH and FSH, which begins the reproductive cycle (ovarian and uterine cycles) again. If fertilisation and implantation occur (pregnancy), levels of oestrogen and progesterone remain elevated because the blastocyst secretes the hormone human chorionic gonadotrophin (hCG) that maintains the corpus luteum and hormone production. The placenta takes over producing oestradiol and progesterone by 6–10 weeks gestation.


Embryology is described as the study of ‘the origin and development of a human being from a zygote to the birth’. (Moore & Persaud 2008, p 2). Moore and Persaud (2008) emphasise that development is a continuum that starts at fertilisation, includes birth—a dramatic event resulting in a change in environment—and continues after birth, with development of teeth, bones, reproductive structures and so on. The process finishes by about the age of 25 years.

This section is limited to antenatal development, which begins with conception. Conception refers to fertilisation that results in pregnancy (Blackburn 2007). The probability of a viable conception has been calculated at only 30% per menstrual cycle (Blackburn 2007), with spontaneous abortions resulting from a range of abnormalities. The processes needed to produce a viable oocyte and a viable sperm have been described earlier in this chapter. This section examines fertilisation and the factors that contribute to successful fertilisation, implantation, development of the embryo and fetus, and development of the placenta.

Jun 18, 2016 | Posted by in MIDWIFERY | Comments Off on The physiology of conception and pregnancy

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