Blood Supply and Changes in Uterus During Pregnancy

Study Notes

The blood supply to the uterus increases significantly during pregnancy, from a pre-pregnancy rate of 10mls/min to 800-900mls/min at 40 weeks of gestation.
This represents approximately 20% of the pregnant woman’s cardiac output.
Blood volume increases by 40-50% during pregnancy, rising from 5 liters to 7.5 liters, with a corresponding 20% increase in red cell mass by the end of pregnancy.

The size of the uterus increases during pregnancy, reaching 30 x 23 x 20cm at term.
Its weight increases to 900gms due to hypertrophy (cell enlargement) and hyperplasia (increased cell number) under the influence of estrogen.

### Changes in Shape

In the first 10 weeks of pregnancy, the isthmus elongates, becoming the lower segment with the globular uterus sitting on top. This creates a characteristic pear shape.
After 20 weeks, uterine muscle tissue stretches to accommodate the growing fetus, aided by the relaxing effect of progesterone.
By 36 weeks, the lower segment of the uterus measures 8-10cms.

The uterus has three layers of muscle fibers:
- Inner circular layer: surrounds the cornua, lower uterine segment and cervix.
- Middle layer: oblique, crisscross arrangement involved in contractions to expel the fetus.
- Outer longitudinal layer: contracts and retracts thickening the upper segment.

Uterus: enlarges, with a thickening of the fundal area. It reaches the umbilicus by 20 weeks and the xyphoid process by 36 weeks. Braxton-Hicks contractions are irregular and begin after 4 months of pregnancy.
Cervix: Forms a mucous plug due to increased cervical mucosa and glandular function; undergoes softening after 3 months (Goodell's sign) and increased vascularity (Chadwick's sign).
Ovaries: Progesterone and estrogen production is taken over by the placenta after 11 weeks.

The cervix does not change in length during pregnancy, but its width increases.
Softening occurs after the third month due to estrogen and increased vascularity.
Cervical mucosa increases along with glandular function to create a mucous plug.

Metabolical changes are driven by increased hormone levels: estrogen, progesterone, and relaxin.
Insulin levels increase during pregnancy.
Caloric requirements increase by approximately 300 calories per day, along with increased needs for protein and fluids.

The vagina, perineum, and vulva experience increased vascularity and hyperemia resulting in a characteristic violet color (Chadwick’s sign).
There is also increased vaginal secretion and a lengthening of the vaginal wall.

Digestive processes slow down during pregnancy, with intestines displaced upwards and to the sides.
Progesterone relaxes the smooth muscle of the stomach, leading to potential heartburn.
Common complications include constipation and hemorrhoids.
Nausea and vomiting (morning sickness) are also common during pregnancy.

Basal metabolic rate (BMR) increases by 20-25% during pregnancy.
Recommended weight gain is 25-35 lbs for women with a healthy BMI, with adjustments for overweight (15-25 lbs) and underweight (25-35 lbs) individuals.
There is an increased need for iron, calcium, fat, and protein during pregnancy.

The digestive system slows due to progesterone, leading to nausea and vomiting, increased salivation, and heartburn.
Displacement of the stomach and intestines can also contribute to these discomforts.
Prolonged gallbladder emptying may lead to gallstones.

The displacement of the stomach and intestines forces the appendix to relocate to the right flank.
Slowed gastric emptying and intestinal transit times are a result of hormonal and mechanical factors.
Pyrosis (heartburn) is common due to reflux of secretions.
Hemorrhoids are caused by increased pressure in veins.

During pregnancy, respiratory rate increases, along with oxygen consumption.
This can lead to nasal stuffiness and nosebleeds due to increased vascular swelling in the nose.

Oxygen consumption increases by 15-20%, with 50% of this increase dedicated to the uterus.
Despite the increased oxygen needs, the increased cardiac output and alveolar ventilation ensure that oxygen consumption exceeds requirements.
Consequently, the arteriovenous oxygen difference decreases, and arterial PCO2 falls.