In this first chapter, we will outline the necessary steps to achieve pregnancy. These include the three primary requirements that have to be fulfilled for an embryo to be produced, and the process for that embryo to implant and grow within the womb (uterus).
Basic Fertility Factors
For a pregnancy to occur, three basic fertility factors need to be fulfilled (Figure 1.1). First, an adequate number of normal sperm have to be deposited in the correct place at the right time. Second, eggs (oocytes) have to be released (ovulation) (this is described in detail in Chapter 3). Third, the passages – the uterus and tubes (Fallopian tubes) – have to be open and normal to allow the transport of sperm up, eggs down and the resulting embryo (after fertilisation) down the tubes and into the uterus. It is in the uterus that the embryo can implant and pregnancy is established.
When a couple who have difficulty conceiving are being investigated, these three basic fertility factors need to be considered.
How Do We Know If Adequate Sperm Are Being Ejaculated?
The correct number of normal sperm
Sperm is the abbreviated term that can be used for both the singular and plural:spermatozoon is one sperm and spermatozoa is more than one sperm and so is plural.
Figure 1.2 illustrates the testis and its duct system. The testis is made up of several lobules, each of which contains coiled seminiferous tubules (within which the sperm are produced), which all drain into the epididymis (the collecting duct). This then flows into the vas deferens, which is the main channel for the outflow of sperm within the semen. There are several hundred tubules in each testis. The tubules are surrounded by a capsule of fibrous tissue (tunica albuginea), which maintains their shape and structure. Within the tubules, the sperm cells mature as they progress from the base of the tubule towards the lumen. Figure 1.2 shows a cross-section of one of the tubules demonstrating sperm maturation, which continues to progress during their passage down the epididymis.
To assess the quality of sperm being ejaculated, a semen analysis is performed on an ejaculated specimen produced by masturbation (not interrupted intercourse) into a sterile pathology sample container. There are two reasons why interrupted intercourse is not suitable: First, some of the semen may be lost; second, the vaginal secretions that are acidic and harmful to the spermatozoa may contaminate the sample. Ideally, there should be 48 hours of abstinence from ejaculation prior to collecting the semen sample to be analysed. The sample should be kept at between room and body temperature until it is delivered to the laboratory, preferably within an hour. It should then be examined as soon as possible. The first assessment is the volume of semen produced (the ejaculate). The normal volume is 2–5 ml. The ejaculated semen contains spermatozoa, but most of the ejaculate is composed of secretions from the accessory glands: the prostate gland and the seminal vesicles.
Next, the specimen is investigated with a special device called a Makler chamber. This is a simple-to-use device, which enables accurate sperm count and motility and morphology evaluation from an undiluted semen specimen. The Makler chamber is composed of two parts:
A lower part has a metal base and includes a flat disc made of optical flat glass. on which the sample is placed. An upper part is made up of the cover glass and is encircled by a metal ring. At the centre of the optical glass is a 1 mm grid, subdivided into 100 squares, each one measuring 0.1 × 0.1 mm. When the cover glass is placed on top, a row of 10 squares measures exactly one millionth of a millilitre (ml). Therefore, the number of sperm heads in 10 squares indicates their concentration in million per ml.
The minimum concentration for a specimen to be called ‘fertile’ should be greater than 14 million sperm per ml. Anything less than this is called oligospermia.
The next parameter assessed is the percentage of sperm that are moving (motility) and this should be at least 40%.
The final parameter is the assessment of sperm shapes (morphology). This has to be done on a dried specimen on a glass microscope slide, treated with special dye and assessed under the microscope. Like people, sperm come in all shapes and sizes. To be considered a ‘normal’ sample, at least 4% of sperm present need to be the perfect shape in head, body and tail (Figure 1.2B). It is usual to assess at least two semen samples at least a couple of months apart.
Another factor that is assessed in specialised fertility laboratories is the presence or absence of antisperm antibodies in the seminal fluid (semen). Normally the sperm are confined to the ducts of the male reproductive system, and are not exposed to the immune system. This is called the blood–testis barrier. However, if sperm do escape due to trauma of the testicles, inflammation or surgery (most commonly a vasectomy), the immune system will produce antibodies to sperm, which it recognises as foreign material. This process is similar to developing antibodies against infective organisms. The antibodies are small enough to get across the blood–testis barrier and attack the spermatozoa in the duct system. This slows or immobilises the spermatozoa, impeding their ability to swim up the passages after ejaculation in the vagina. To understand this, imagine the sperm as swimmers doing laps in a pool and the antibodies as a group of playful children who jump into the pool and grab the arms and legs of the swimmers. Some of the swimmers can still complete their laps, but fewer will do so, and at a slower rate. Consequently, the presence of antisperm antibodies can be a barrier to conception.
In the correct place
It is important that sperm are deposited high into the vagina so that sperm quickly migrate into the neck (cervix) of the uterus.
This is important because the vaginal secretions are acidic and so are toxic to sperm. Once the sperm are within the cervix, they are protected from acidity by the cervical mucus and can survive for several days. Although pregnancies have resulted from sperm being ejaculated at the opening of the vagina, without vaginal penetration, the chance of conceiving is far higher if the ejaculation is high into the vagina.
Assessment of adequate technique can be obtained by asking about coital (sexual intercourse) technique such as the adequacy of erections, the sensation of ejaculation for the man and how the ejaculate feels to the woman in the vagina. This reassures the couple that ejaculation has taken place into the vagina. It is normal for some of the ejaculate to ooze out of the vagina when the woman moves, but this does not prevent conception. Some of the ejaculated sperm enter the cervical mucus within seconds, and these are the ones that will be responsible for fertilisation. Coital technique is not important unless the penis is totally misplaced or has structural abnormalities such as an opening under the shaft rather than its tip (hypospadias).
It is also possible (although rarely done) to examine the cervical mucus under the microscope after intercourse and confirm that there are sperm in the specimen. This is called a postcoital test (also known as the Sims-Huhner test, which is discussed in more detail in Chapter 5). However, this test is rarely done these days.
At the right time
The released oocyte can only be fertilised for a few hours after ovulation, so the best time for intercourse is just before ovulation. Sperm ejaculated into the vagina and protected by cervical mucus can live for a few days. They are then released in batches to travel up the passages, and may encounter an oocyte that is ready to be fertilised. It is therefore important that sperm are deposited in the vagina every couple of days around the time of ovulation. As such, couples are advised have intercourse at least every second day over the fertile week, which is described later in this chapter. There is no benefit in more frequent intercourse, and in fact some men may have difficulty building up adequate numbers of fertile sperm with more frequent ejaculations. As couples may have difficulty remembering when they have had intercourse, it can be recorded on a temperature chart (discussed in Chapter 3).
There are many myths about timing intercourse to predetermine whether a boy or girl will be conceived, including special diets and intercourse positions and restricting timing to a specific day with respect to ovulation, popularised by Dr Landrum Shettles in America in the 1960s. There is no scientific basis for these, nor any evidence that they work.
Eggs Have to Be Produced: The Basics of Ovulation
The most obvious indication of ovulation is the presence of regular menstrual cycles of between 25 and 32 days in duration when counting from day one of one bleed to day one of the next bleed; the start of the next cycle.
Normal menstruation lasts up to seven days, without passing clots or flooding.
Ovulation usually occurs about 14 days before the next period (menstruation, or menses), and, in regular cycles, that is between day 11 and day 18 after the start of menstruation (the fertile week). This is the time when intercourse should take place at least every second day to maximise the chance of conception. Women who have irregular and longer cycles are either not ovulating or are ovulating irregularly, and need hormonal stimulation (ovulation induction [OI]) to regulate ovulation and improve the chance of becoming pregnant (see Chapter 3). After ovulation, the released oocyte enters the outer end of the Fallopian tube, called the ampulla, where it is surrounded by sperm (see Figure 1.3). Millions of sperm are deposited into the vagina. Hundreds of thousands of these will pass through the cervix, a few thousand will enter the Fallopian tubes and a few hundred will eventually reach the oocyte in the ampulla. Only one of these will enter the oocyte to fertilise it. When a sperm penetrates the oocyte, the oocyte’s shell prevents any further sperm entering.
Figure 1.3 An ooocyte surrounded by many sperm.
After sperm-penetration fertilisation occurs, an early embryo develops. Both oocytes and sperm have 23 chromosomes each: one copy each of numbers 1–22, with the oocyte only ever having X chromosomes, and the sperm having either an X or Y chromosome as its twenty-third. If the fertilised embryo has an XY complement, it will produce a male child, whereas if it has two X chromosomes (XX), the child will be female. If the oocyte has not closed its shell after a sperm has penetrated it, and a second sperm enters (polyspermy), the embryo will have 69 chromosomes, which is not compatible with life .
Fertilisation
Fertilisation requires the joining of the sperm and the oocyte, which takes place in the ampulla of the Fallopian tube.
There are four stages of the fertilisation process.
First, the sperm has to be activated. At ejaculation, sperm are not capable of fertilising an oocyte. It is only after mixing with vaginal secretions that they undergo changes, which are collectively known as capacitation, and give them the ability to bind to and penetrate the glycoprotein covering of the oocyte.
Second, the sperm (a fast-moving small cell with a head and midpiece measuring about 5 µm) has to bind to the much larger and immobile oocyte (about 100 µm in diameter). Figure 1.3 shows an oocyte surrounded by many sperm, one of which eventually binds to the oocyte as the ‘chosen sperm’. The binding of the oocyte and the sperm occurs with attraction between their membranes through the thick zona pellucida (ZP) of the oocyte. The binding is made possible when receptors on the oocyte and the sperm meet. This initiates the acrosome reaction, in which the sperm releases chemicals, which can bore a hole in the ZP.
Third, the two cells have to fuse together. The tail of the sperm is immobilised and its head and body are pulled into the interior substance of the oocyte (cytoplasm). The chromosomes of the sperm are included in a new membrane envelope, forming the male pronucleus (see Figure 1.4).
Figure 1.4 Embryo development from a fertilised oocyte to blastocyst stage.
Fourth, the sperm and oocyte chromosomes have to be joined together by the fusion of their covering membranes. The resultant embryo is initially called a zygote (see the top left of Figure 1.4), and it has a full set of 46 chromosomes: 22 pairs of autosomes plus either X and Y, or X and X.
Once the oocyte is fertilised, the ZP prevents additional sperm from penetrating. If this fails and polyspermy occurs, the zygote will have 69 chromosomes, which, as discussed earlier in this chapter, is not compatible with life.
The Passages (Uterus and Tubes) Have to Be Open and Normal
The third basic fertility factor for conception is that the tubes and uterus need to be open and ‘normal’ (this is discussed in detail in Chapter 4) to allow the oocyte and sperm to unite in the ampulla and then be transported to the uterine cavity. During this time, the early embryo starts to divide, comprising 4–8 cells by day three after ovulation. The cells in the embryo keep dividing as it is transported to the uterine cavity, by which time it is five days old and consists of about 100 cells (see Figure 1.4). During this rapid growth, the Fallopian tube provides nutrition until the embryo establishes its blood supply with the development of the placenta (afterbirth). As such, the Fallopian tubes are not just ‘a piece of plumbing’ connecting the uterus to the ovary, but complex structures responsible for transporting the sperm and oocytes (gametes), and then the embryo, and providing the only source of nutrition to the embryo for its rapid cell division and development during the four to five days it takes to reach the uterus. The lining (epithelium) of the tubes is intricate, being covered by fine hairs (cilia), which beat in a co-ordinated rhythm to sweep the sperm, oocytes and the embryo along its surface (see Figure 4.6 in Chapter 4).
The final part of the passages is the uterine cavity, which has to facilitate the implantation of the developing embryo, helping it to establish its blood supply via the early placenta. The presence of inflammation, scarring or polyps or fibroids in the uterine cavity could all prevent this.
The survival and implantation of the early embryo is dependent on the inner lining of the uterus (endometrium) being maintained, which requires the corpus luteum in the ovary to continue secreting the female sex hormones oestrogen and progesterone, which stimulate the endometrium to be retained until the placenta takes over at about three months into the pregnancy.
Implantation
Successful implantation requires a healthy blastocyst, a receptive endometrium and effective communication between the two.
The endometrium is only receptive for a few days – called the window of implantation – after which implantation is not possible.
When the developing embryo is about five days old and has multiplied to about 100 cells, it reaches what is called the blastocyst stage (see Figure 1.4), which is when it attaches to the endometrium, and the epithelium secretes a number of biochemical substances (called enzymes), which are important for implantation. The endometrium of the uterine develops protrusions called pinopodes, which help the blastocyst adhere to the endometrial lining. The attachment of the blastocyst is enhanced by the enzymes that are secreted by the endometrium, and a pathway for absorbing nutrients is established as the early placenta. During this time, the rapidly dividing cells of the blastocyst divide into two distinct layers called the cytotrophoblasts and the syncytiotrophoblasts. The inner layer (cytotrophoblasts) will form the embryo, while the outer layer (syncytiotrophoblasts) will form the placenta and membranes surrounding the fetus.
A successful pregnancy requires that the endometrial tissue have sufficient exposure to progesterone to support the endometrial lining and prevent expulsion of the trophoblasts. The syncitiotrophoblast cells secrete a hormone called human chorionic gonadotrophin (HCG). HCG acts as a message to the corpus luteum to keep it functioning, enabling the continued production of progesterone. Until the placenta fully develops after two to three months, the corpus luteum is the source of progesterone and oestrogen for the developing pregnancy. The hormones stimulate the cells of the endometrium to accumulate nutrients and the development of blood vessels to support the blastocyst. As fetal blood vessels develop, the mother’s uterine arteries develop spiral arterioles, which form the feto-maternal circulation, which allows nutrients and waste through.
Finally, rather than passively accepting the attaching embryo, the endometrium appears to play an active role in blastocyst selection. This is evident by abnormal embryos triggering a chemical signal within the endometrium, which then has the ability to respond to that signal by engulfing the blastocyst and destroying it.
Age and Conceiving
A woman’s fertility gradually declines in her thirties, particularly after age 35 (see Figure 1.5). At peak fertility, a healthy, fertile woman has a 25% chance of conceiving each month when she has well-timed intercourse with a fertile man.
Figure 1.5 The Age Factor.
By age 40, a woman’s chance of conception is less than 5% per menstrual cycle, which means that fewer than 5 out of every 100 women are successful after each month of trying to conceive.
Women do not remain fertile until the menopause. Most women become unable to have a successful pregnancy some time in their mid-forties, although they may continue to menstruate. This is not only due to the number of oocytes available but the quality of oocytes also gradually declines. As women age, there are more oocytes with abnormal chromosomes, but even those that are normal chromosomally have lower energy with fewer mitochondria (which are the power generators of the oocytes) and therefore are less likely to produce healthy embryos. Although couples believe that they can use fertility treatments such as in vitro fertilisation (IVF), a woman’s age also affects the success rates of infertility treatments, with success declining with age.
When and How to Get Help
Most couples who want to conceive do so within 12 months of unprotected regular intercourse. So, if a couple have not conceived after a year, it is time to get some expert advice. Women in their mid-thirties or older are recommended to do so after six months of trying to conceive. The first point of call should be the family doctor or general practitioner (GP). The GP should obtain a history, organise some early investigations and treat the consultation as ‘pre-pregnancy counselling’ with regards to advice about diet, alcohol, smoking and weight. Blood tests ensuring that immunity to German measles (rubella) and chickenpox (varicella) is present. The GP should then refer the couple to a specialist.
A Systematic Approach to Investigations and Treatment
To advocate a logical and systematic approach to investigating and treating a couple with subfertility, an algorithm (flow chart) has been developed by the author (see Figure 1.6).
Figure 1.6 An algorithm for the management of a couple with subfertility.
The crux is that virtually anyone can conceive these days by using IVF, OI, egg or sperm donation, or surrogacy: often referred to collectively as assisted reproductive technology (ART).
