Pregnancy is the period from conception to birth. Pregnancy begins with fertilization of an ovum (egg) and its implantation. The egg develops into the embryo, which becomes the fetus, and the placenta, the organ that nourishes the embryo and then the fetus. Most eggs implant into the uterus. Very occasionally, an egg implants into an abnormal site, such as a fallopian tube, resulting in an ectopic pregnancy.

Stages and features of pregnancy 

Pregnancy is traditionally dated from the first day of a woman’ s last menstrual period (LMP), although conception would not have taken place until two weeks after this. A normal pregnancy is considered to last between 37 and 42 weeks; it is divided into three stages (trimesters) of three months each. For the first eight weeks of pregnancy, the developing baby is called an embryo; thereafter it is called a fetus. In the first trimester the breasts start to swell and may become tender. Morning sickness is common. The baby’ s major organs have developed by the end of this stage. During the second trimester, the mother’ s nipples enlarge and darken and weight rises rapidly. The baby is usually felt moving by 16–22 weeks. During the third trimester, stretch marks and colostrum (nipple secretions) may appear, and Braxton Hicks’contractions may be felt. The baby’ s head engages at about 36 weeks. 

First trimester (0 to 12 weeks)

The first sign of pregnancy is usually the absence of a menstrual period, although some women have breakthrough bleeding. The breasts start to swell and may become tender as the mammary glands develop to prepare for breast-feeding. The nipples start to enlarge and the veins over the surface of the breasts become more prominent. A supportive bra should be worn. Nausea and vomiting are common, and are often worse in the morning; these problems usually persist for six to eight weeks. There is a need to pass urine more frequently and there is often a creamy white discharge from the vagina. Many women feel unusually tired during the early weeks of pregnancy. Some notice a metallic taste in the mouth or a craving for certain foods or non-edible products. Weight begins to increase towards the end of this stage

Second trimester (13 to 28 weeks)

From 16 weeks, the enlarging uterus is easily felt from the outside and the woman begins to look noticeably pregnant. The nipples enlarge and darken, and skin pigmentation may deepen. Some women may feel warm and flushed. Appetite tends to increase and weight rises rapidly. The woman’s facial features tend to become heavier. By 22 weeks (and usually between the 16th and 20th weeks), most pregnant women have felt the baby moving around (a sensation that is sometimes known as “quickening”). During the second trimester, nausea, vomiting, and frequency of urination diminish, and the woman may feel generally better and more energetic than she did during the early weeks. The heart-rate increases, as does the volume of blood pumped by the heart; these changes allow the fetus to develop properly. However, they put an extra strain on the heart of women who have pre-existing heart disease

Third trimester (29 to 40 weeks)

In some women, stretch marks develop on the abdomen, breasts, and thighs. Also, a dark line may appear that runs from the umbilicus to the pubic hair (known medically as linea nigra). Colostrum (secretions from the nipples) can be expressed from the nipples. Minor problems are common. Many women become hot and sweat easily because body temperature rises slightly. More rest may be needed at this stage, although for many women it is difficult to find a comfortable position. Braxton Hicks’contractions may start to get stronger. The baby’s head engages (drops down low into the pelvis) around the 36th week in a first pregnancy, but not until a few weeks later in subsequent pregnancies. This so-called “lightening”may relieve pressure on the upper abdomen and on breathing, but it increases pressure on the bladder and may result in a more copious vaginal discharge.

Hormonal changes and their effects in pregnancy

A pregnant woman undergoes many changes that enable her to maintain the pregnancy, nourish the baby, and prepare for breast-feeding. These adaptations are brought about by increased levels of the female sex hormones oestrogen and progesterone, and by the action of two other hormones, human chorionic gonadotrophin (HCG) and human placental lactogen (HPL), produced only by the placenta.


  • Decreases the excitability of smooth muscle, thereby helping to prevent uterine contractions and premature labour.
  • Causes constipation and oesophageal acid reflux as a result of its effects on smooth muscle.
  • Increases body temperature. 
  • Affects mood.
  • Increases breathing rate.

Human placental lactogen (HPL)

  • Increases energy production necessary for fetal development. 
  • Causes enlargement of breasts and development of milk glands.
  • Induces temporary diabetes mellitus (gestational diabetes) in susceptible women as a result of its effects on metabolism. 

Human chorionic gonadotrophin (HCG)

  • Increases energy production necessary for fetal development.
  • Induces gestational diabetes in susceptible women. 


  • Are important for the development of the reproductive system and breasts.
  • Stimulate growth of the uterine muscle to enable the powerful contractions of labour. Increase vaginal secretions.
  • Increase the size of the nipples and help the development of milk glands in the breasts.
  • Increase the production of protein, which is essential for healthy growth of the woman and fetus.
  • Alter collagen and other substances to allow body tissues to soften and stretch in preparation for labour.
  • Relax ligaments and joints.
  • May cause sciatica and backache, and may also contribute to the formation of varicose veins as a result of their effects on body tissue. 

Melanocyte-stimulating hormone 

  • Stimulates pigmentation (in combination with oestrogens), particularly of the nipples.
  • May also produce chloasma (darkening of the facial skin)

Problems in pregnancy 

In addition to the expected features of pregnancy, such as experiencing nausea and tiredness, some women experience certain common, minor health problems. Although these conditions may be uncomfortable or troublesome, they usually disappear after delivery. During pregnancy, food passes through the intestine more slowly than normal. This enables more nutrients to be absorbed for the baby, but also tends to cause constipation. Pica (craving to eat substances other than foods) is another common condition. In late pregnancy, the growing fetus puts pressure on the internal organs; this pressure may result in haemorrhoids, heartburn (due to acid reflux), swollen ankles, and varicose veins. Other common disorders during pregnancy include urinary tract infections, stress incontinence, urinary), and vaginal candidiasis. Complications of pregnancy and disorders that affect it include antepartum haemorrhage; diabetic pregnancy; miscarriage; polyhydramnios; pre-eclampsia; prematurity; and Rhesus incompatibility. 

Multiple pregnancy

More than one fetus in the uterus. Multiple pregnancy can occur if two or more ova (eggs) are fertilized at the same time or if a single egg divides early in development.

Causes and incidence 

Twins are the most common type of multiple pregnancy and occur in about 1 in 80 pregnancies; triplets occur in about 1 in 8,000, and quadruplets in about 1 in 500,000. Multiple pregnancies are more common in women who are treated with fertility drugs, or if a number of fertilized ova are implanted in the uterus during in vitro fertilization

Symptoms and complications

The common problems that are associated with normal pregnancy may be more severe in a multiple pregnancy. There is also an increased risk of severe morning sickness, anaemia, and ante-partum haemorrhage. Hypertension (high blood pressure), polyhydramnios (an excess of amniotic fluid), postpartum haemorrhage, and malpresentation (abnormal positioning of a fetus just before birth) all occur more frequently in a multiple pregnancy. Prematurity is a common complication, and the weight of each baby is usually less than the weight of a single baby. Caesarean section is required more frequently than for single pregnancies.

Diagnosis and treatment

During the woman’ s antenatal examination, the doctor or midwife may be able to feel more than one fetus, and may find that the abdomen is larger than expected for the stage of gestation. The doctor or midwife may also be able to hear more than one fetal heartbeat when listening through a stethoscope. Ultrasound scanning may be used to confirm the diagnosis. The woman is advised to take more rest during pregnancy and to increase her intake of protein. Supplements of iron and folic acid are also recommended in order to prevent iron-deficiency anaemia in the mother and neural tube defects in the fetuses, respectively.

Pregnancy tests 

Tests on urine or blood performed to determine whether a woman is pregnant. Pregnancy testing kits are available over the counter from pharmacies. All kits are designed to test for the presence of human chorionic gonado-trophin (HCG) in a sample of urine. This hormone is normally produced only by a developing placenta; the tests are extremely accurate (about 97 per cent accurate for a positive result and 80 per cent accurate for a negative result) and can be reliably used from the first day of a missed period. The test can, however, misdiagnose pregnancy in cases of hydatidiform mole, due to very high HCG levels. Different brands of test vary, but all involve introducing a sample of urine to a test stick that has been treated with a chemical that reacts with HCG. Blood tests for detecting pregnancy can be used from 9 to 12 days after conception (even before a period is missed).

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