Pituitary gland

In pregnancy the pituitary gland increases in size by 50%. The secretion of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) falls to very low levels, whereas that of adrenocorticotrophic hormone (ACTH), melanocyte hormone and prolactin increases. Prolactin levels, for example, increase until the 30th gestational week and then more slowly to term. Prolactin may be a factor in the fall of FSH and LH to very low levels by the eighth week of pregnancy.

Adrenal gland

Total corticosteroids increase progressively to term. This could, to some extent, account for a pregnant woman’s tendency to develop abdominal striae, glycosuria and hypertension.

Thyroid gland

The thyroid gland enlarges during pregnancy, occasionally to twice its normal size. This is due mainly to colloid deposition caused by a lower plasma level of iodine, consequent on the increased ability of the kidneys to excrete during pregnancy. Oestrogen stimulates an increased secretion of thyroxine-binding globulin. In consequence both T₃ and T₄ levels rise. These raised levels do not indicate hyperthyroidism, as both the thyroid-stimulating hormone (TSH) and the free thyroxine levels are in the normal range. When tests are made to determine thyroid function these changes must be taken into account.

Uterus

The effect of the hormonal stimulation is most marked upon the tissues of the genital tract, and the uterine muscle fibres grow to 15 times their prepregnancy length during pregnancy, whereas uterine weight increases from 50 g before pregnancy to 950 g at term (Fig. 5.1). In the early weeks of pregnancy the growth is by hyperplasia, and more particularly by hypertrophy of the muscle fibres, with the result that the uterus becomes a thick-walled spherical organ. From the 20th week growth almost ceases and the uterus expands by distension, the stretching of the muscle fibres being due to the mechanical effect of the growing fetus. With distension the wall of the uterus becomes thinner and the shape cylindrical (Fig. 5.2). The uterine blood vessels also undergo hypertrophy and become increasingly coiled in the first half of pregnancy, but no further growth occurs after this, and the additional length required to match the continuing uterine distension is obtained by uncoiling the vessels.

Fig. 5.1 Relation of the weight of the uterus and the thickness of the uterine wall to the weight of the fetus at varying times in normal pregnancy.

Fig. 5.2 Enlargement of the uterus in normal pregnancy.

The uterus is derived from the two Müllerian ducts and the myometrium is made up of a thin external, largely longitudinal, layer; a thin inner, largely circular layer; and a thick, intricately interlaced middle layer, which comprises two spiral systems of interdigitating muscles derived from the two Müllerian ducts. The proportion of muscle to connective tissue is greatest in the fundal area and diminishes as the lower segment of the uterus and cervix is approached, the lower half of the cervix having no more than 10% of muscle tissue.

The effect of the uterine distension is to stretch both interdigitating spiral systems and to increase the angle of crossing of the fibres, in the thinner lower segment area where the fibres cross at an angle of about 160 ° and are less stretched. Incision of the myometrium in this zone is anatomically more suitable, and experience of lower segment caesarean section confirms that healing is better (Fig. 5.3).

Fig. 5.3 Representation of the obliquity of decussation and interweaving of myometrial fibres. The obtuse angle of decussation in the lower segment can be seen.