INTRODUCTION
The anatomical, physiological, and hormonal changes in pregnancy are significant and occurr in response to stimuli from the placenta and the fetus. Due to these changes, there are physiological symptoms in first trimester of pregnancy. The understanding of these changes is essential to treat symptomatology of pregnant woman, and also to know the physiological basis for certain conditions of pregnancy. For majority of these complaints, only reassurance may be enough, but for some therapeutic measures may have to be undertaken to ensure good maternal and fetal outcome.
The changes occur in all systems of the body starting from the first trimester and gradually increasing toward the last trimester. Major changes in first trimester occur in the genital system, gastrointestinal system, cardiovascular systems, and central nervous system. Systemic changes, leading to physiological symptoms in first trimester of pregnancy occur from first trimester onward (Box 1). The major factors responsible for the physiological changes in pregnancy are increasing levels of human chorionic gonadotropin (hCG), estrogen, and progesterone.
GENITAL SYSTEM
Increased level of progesterone is associated with increased vascularity of pelvic organs and decreased vascular resistance. This leads to congestion of genital organs.1
Uterus
Uterine size is increased both due to intrauterine growth of the gestational sac (distension), and also due to myohyperplasia and hypertrophy of myometrium under the influence of estrogen. Progesterone excess is associated with increased vascularity.2
The shape of the pre-pregnant uterus is pyriform which becomes globular by end of the first trimester and then it again changes to oval, from 12 weeks onward. Due to increasing tension in the growing amniotic sac, there is downward pressure on the cervix.
Uterine Signs
- Size, shape, and consistency: The uterus is enlarged to the size of hen's egg at 6th week, size of a cricket ball at 8th week, and size of a fetal head by 12th week. The pyriform shape of the non-pregnant uterus becomes globular by 12 weeks. The uterus becomes acutely anteverted between 6 weeks and 8 weeks. There may be a symmetrical enlargement of the uterus if there is lateral implantation. This is called Piskacek's sign where one half is more firm than the other half. As pregnancy advances, symmetry is restored. The pregnant uterus feels soft and elastic
- Hegar's sign: It is present in two-thirds of cases. It can be demonstrated between 6 weeks and 10 weeks, a little earlier in multiparae. This sign is based on the fact that: (1) Upper part of the body of the uterus is enlarged by the growing fetus, (2) lower part of the body is empty and extremely soft, and (3) the cervix is comparatively firm. Because of variation in consistency, on bimanual examination (two fingers in the anterior fornix and the abdominal fingers behind the uterus), the abdominal and vaginal fingers seem to appose below the body of the uterus
- Palmer's sign: Regular and rhythmic uterine contraction can be elicited during bimanual examination as early as 4–8 weeks. Palmer in 1949, first described it and it is a valuable sign when elicited.
Cervix
Congestion and softening of cervix occurs during early trimester. Non-pregnant cervix has a firm feel on touch but, during pregnancy it is soft. Increased vascularity causes congestion of cervix giving rise to bluish discoloration of cervix and is known as Goodell's sign. During the first trimester, isthmus elongates to three times original length and after 12 weeks it unfolds from above downward. Thus, lower segment starts to form from the end of the 12th week. If the circular fibers of the internal os are weak then the abortion takes place due to incompetent cervix.
Vagina
Vaginal mucosa appears bluish and congested due to increased vascularization, this leads to excessive non-purulent vaginal discharge (physiological leucorrhea). There is increased pulsation, felt through the lateral fornices at 8th week called Osiander's sign. Similar pulsation is, however, felt in acute pelvic inflammation.
External Genitalia
A dusky view of vestibule and anterior vaginal wall usually seen in multipare is known as Chadwick's or Jacquemier's Sign and is due to altered vascularity.
Ovaries
Ovulation ceases during pregnancy and the maturation of new follicles is suspended. A single corpus luteum of pregnancy may be found in the ovary of pregnant women and functions maximally during the first 6–7 weeks of pregnancy.
Breast
Breast changes are evident in primigravidas. There is deeper pigmentation of the areola and nipples are larger and erectile. The breast changes are evident between 6 weeks and 8 weeks. There is enlargement with vascular engorgement evidenced by the delicate veins visible under the skin. The nipple and the areola (primary) become 3more pigmented specially in dark women. Montgomery's tubercles are prominent. Thick yellowish secretion (colostrum) can be expressed as early as 12th week.
GASTROINTESTINAL SYSTEM
Morning sickness is a common complaint in the first trimester and its severity very well correlates with level of hCG. Relaxation of the cardiac sphincter of stomach causes regurgitation of food and leads to recurrent vomiting and retrosternal burning in early trimester. Under the influence of progesterone, there is decreased gastrointestinal motility and a decreased muscle tone of the intestinal tract which is responsible for anorexia, indigestion, and constipation during pregnancy. Liver function is depressed during pregnancy but there are no changes in the liver function test. There is delayed emptying of gall bladder.
URINARY SYSTEM
Enlarged size of the uterus along with its exaggerated anteverted position leads to frequency of urine due to bladder irritability. This may also be due to congestion of the bladder mucosa.
CARDIOVASCULAR SYSTEM
Effect of hormonal changes on the cardiovascular system leads to hyperdynamic circulation. There is relaxation of smooth muscles of vessels leading to decreased vascular resistance in almost all vasculature. This effect is measured as overall fall of diastolic blood pressure and mean arterial blood pressure by 5–10 mm of Hg. The cardiac output starts rising since 5 week of pregnancy.2 Blood volume starts rising from 10th week onward. All these changes in the cardiovascular system are responsible for complaints like giddiness, weakness, headache, and heaviness in the head.3
MUSCULOSKELETAL SYSTEM
During early weeks of pregnancy, there is secretion of relaxin. Under the influence of relaxin, there is relaxation in joint synovial membranes leading to instability of synovial joints like sacroiliac joint and pubic symphysis. Usually, there is no movement in these joints, but because of these changes, there is instability in the pelvis leading to pain in the hips during walking, and turning while in lying down position.4 Pregnant women commonly complain of cramps in the legs and calf muscle pain, which may be due to decreased availability of energy resources like adenosine triphosphate.
CENTRAL NERVOUS SYSTEM
Increased level of hormones may have effect on central nervous system causing nausea and vomiting.
CUTANEOUS CHANGES
Hyperdynamic circulation in pregnancy leads to increased vascularity of the skin during pregnancy and disturbed thermoregulation of the body, leading to rise in basal body temperature by 1°F. Due to this, pregnant women complain of heat intolerance.
WEIGHT
In the first trimester, a woman may lose weight because of nausea, vomiting, and anorexia
OSMOREGULATION
During pregnancy, there is increased sodium retention due to estrogen, progesterone, aldosterone, and antidiuretic hormone. Increased accumulation of fluid leads to decrease in colloid osmotic pressure due to hemodilution.4
METABOLISM
Initially during the first trimester, there is negative protein metabolism and lipolysis. Gradually, as symptoms of early pregnancy subside, protein synthesis and lipogenesis develop due to estrogen effect.
ENDOCRINE SYSTEM
Before the placental function starts corpus luteum acts as a rescue till 6–8 weeks of pregnancy. Syncytiotrophoblasts secrete a number of protein and steroidal hormones that simulate pituitary hormones.5 Some of the important hormones are:
- Human chorionic gonadotropin: is a glycoprotien hormone which simulates luteinizing hormone, plays a major role in maintenance of pregnancy and immunosuppression. It stimulates the adrenal and placental steroidogenesis, and maternal thyroid gland
- Human placental lactogen: is lactogenic and functions as growth hormone in pregnancy
- Human chorionic thyrotropin
- Human chorionic corticotropin
- Steroidal hormones: estrogen and progesterone start rising since 9th week of pregnancy.
EMBRYONAL AND FETAL DEVELOPMENT
Normal embryonal and fetal development during first trimester is illustrated in table 1. It is amply clear that any insult during this phase may cause first trimester abortion.
Physiological maternal adaptation in pregnancy starts as soon as conception occurs. These changes are necessary for implantation and healthy growth in early pregnancy. The understanding of these changes and influence of age, parity, race, multiple gestation, and other variables has to be understood to appreciate the adaptations and disease process that occur during pregnancy.
REFERENCES
- Ganong WF. The gonads: development and function of reproductive system. In: Ganong WF, editor. Review of Medical Physiology. 2nd ed. McGraw-Hill; PA: Philadelphia, 2009. p.142–7.
- Pandey AK, Banerjee AK, Das A, et al. Evaluation of maternal myocardial performance during normal pregnancy and post partum. Indian Heart J. 2010; 62 (1): 64–7.
- McFadyn IR. Maternal changes in normal pregnancy. In: Turnbull A, Chamberlin G, editors. Obstetrics. 3rd ed. Churchill Livingstone; Edinburgh: 1994. p.151–71.
- Stirrat GM. Physiological changes in pregnancy. In: Stirrat GM, editor. Obstetrics. 2nd ed. Boston: Scientific Publication; Blackwell Oxford, 1986. p.7–22.
- Roti E, Gnudi A, Braverman LE. The placental transport, synthesis and metabolism of hormones and drugs which effect thyroid function. Endocrinal review. 1983; 4 (2): 131–49.