Conception, implantation and placentation
The reproductive organs undergo several characteristic changes during pregnancy. These are mainly a consequence of hormonal changes, increased blood flow and vascular congestion in the pelvis. The anatomy and physiology of the female reproductive organs are described in the book entitled The Gynaecological Examination from the Skills in Medicine series.
Five to six days after conception, which usually takes place in the distal part of the Fallopian tube, the fertilised ovum (zygote) implants in the decidualised endometrium (decidua). Prior to implantation, trophoblast cells develop on the surface of the zygote. These initially produce the hormone human chorionic gonadotropin (hCG), which partly has the same effect on the corpus luteum as luteinising hormone (LH), namely stimulating the production of progesterone and oestrogens. These hormones play a role in the formation and maintenance of the decidua. In addition, hCG ensures the maintenance of the corpus luteum. During, but mainly after, implantation the placenta forms from the trophoblast and the decidua. In about the 12th week of pregnancy the placenta takes over the corpus luteum’s hormonal role. Several objective and subjective changes in the woman during pregnancy are the consequence of these hormonal changes.
The development of the embryo and foetus can be checked using ultrasonography and objective measurements from a few weeks after conception onwards (see “Subsequent check-ups”, Regular check-ups during pregnancy) and subjective symptoms (nausea, tender breasts).
The placenta develops from a part of the trophoblast and the basal decidua (the part of the decidua between the zygote and myometrium). During the course of the pregnancy the placenta develops into a saucer-shaped structure with a diameter of about 20 cm and a weight of 500-600 g. On the maternal side there are a number of grooves and the surface is lobed. The umbilical cord protrudes from the foetal side of the placenta in or near to the centre in most cases. The membranes cover the foetal side of the placenta and, from the edge of the placenta, they envelop the space (amniotic cavity) in which the amniotic fluid and foetus are located.
The placenta plays a crucial role during the course of pregnancy. The hormonal function of the placenta has already been mentioned. Other essential functions are the transport of oxygen and nutrients from the mother to the foetus and the removal of waste substances.
Changes in the reproductive organs
Adnexa
The corpus luteum, which is located in one of the ovaries, plays an essential role at the start of the pregnancy. With the production of oestrogens and progesterone, it maintains the decidualisation of the endometrium, thus preventing rejection of the implanting embryo. Once the placenta has taken over this function, the corpus luteum usually disappears. During the course of the pregnancy the ovaries can increase in size due to the increased blood flow. This increased blood flow is the reason why the fallopian tubes can be somewhat oedematous at the end of the pregnancy.
Uterus
The softening of the cervix, the uterus and, in particular, the area between the cervix and uterus where the large blood vessels to the uterus are attached, is a characteristic change at the start of pregnancy. The strong local increase in blood supply causes this soft and scarcely palpable transition from the cervix to the body of the uterus. This phenomenon is termed Hegar’s sign. Where implantation takes place the external surface of the uterus can initially exhibit a lateral bulge: Piskacek’s sign (see also Chapter 2, Physical diagnostics and laboratory tests).
To accommodate the growing embryo, the uterus increases markedly in size and weight during the pregnancy. Prior to pregnancy the uterus is a pear-shaped, 7 to 10 cm long, firmly palpable organ weighing 80 g with a flattened lumen in the anteroposterior axis. In the non-pregnant state, the anterior and posterior walls of the uterus touch each other so that there is no significant lumen. During pregnancy the uterus changes into a balloon-shaped cavity with a stretched soft wall and an own weight of about 1000 g. At the end of pregnancy the foetus weighs 3000 to 3500 g, the placenta weighs about 500 g and the amniotic cavity contains about 500 to 800 g of amniotic fluid.
In the second half of pregnancy contractions regularly occur in the myometrium (Braxton Hicks contractions). These are experienced by the pregnant woman as the abdomen becoming hard.
Vagina and vulva
The characteristic changes to the vulva and vagina at the start of the pregnancy are the consequence of an increased blood supply and a certain amount of venous congestion. This is evident in the form of a bluish-red (livid) discolouration of the initially pink epithelium. This is referred to as Chadwick’s sign (see also “Physical diagnostics and laboratory tests”).
Frequently the woman also notices increased vaginal discharge. This has several possible causes. The increased blood supply in the lesser pelvis leads to enhanced transudation in the vagina and there is also increased excretion of mucus in the cervical crypts. Due to the constantly high concentration of progesterone and oestrogen, desquamation of the vaginal epithelium increases. A vaginal infection might also be present. The risk of a vaginal Candida infection increases during pregnancy. If no vaginal infection is diagnosed, increased vaginal discharge need not be treated.
Birth canal
During parturition the foetus leaves the uterus and must then pass through the birth canal. This is only possible after changes to the foetus (position and shape of the skull) and the mother’s pelvis.
The birth canal is the J-shaped passage in the sagittal plane leading from the uterus to the outside world. It consists of a part formed from the distal segment of the uterus during parturition, the lowest uterine segment, the cervix and the vagina. The birth canal is made up of soft tissue in the pelvis. The ligaments, muscles and fascia of the suspension apparatus of the uterus and the pelvic floor are likewise composed of soft tissue. In addition to this the ‘bony’ pelvis is also distinguished. This consists of four parts: the sacrum, which is attached to the vertebral column via the sacral promontory; the coccyx, which is connected to the distal end of the sacrum and the two coxal bones on either side of the sacrum. The coxal bones are connected dorsally with the sacrum and ventrally with each other via the pubic bone. The parts that form the bony pelvis have a certain degree of mobility and this movement increases during pregnancy due to the softening of the ligaments. During parturition this increased mobility ensures the foetus passes smoothly through the birth canal. Finally, the internal pelvic shape and dimensions determine the course of the parturition (see “Physical diagnostics and laboratory tests”).