Early Nutrition and Function of the Placenta

Early Nutrition of the Embryo

• Progesterone helps in the conversion of endometrial stromal cells into larger cells, containing nutrients like glycogen, proteins, lipids, and minerals.
• These enlarged cells are called Decidual Cells. The mass of these cells is called Decidua.
• The first week after implantation, the embryo relies solely on the stored nutrients in Decidual cells. This continues until the 8th week.
• After the 16th day after fertilization, the placenta begins to deliver nutrients.

Function of the Placenta

• Trophoblastic cords (form villi) attach to the endometrium.
• Fetal blood capillaries grow into these cords.
• Maternal blood sinuses develop outside the cords.
• Villi with fetal blood, are surrounded by sinuses containing maternal blood.
• The placenta is a combination of fetal and maternal components.
• Nutrients and other substances pass through the placenta mainly by diffusion.

Permeability of the Placenta

• The primary role of the placenta is to facilitate the diffusion of food and oxygen from the mother's blood to the fetus, and the diffusion of waste products in the opposite direction.
• In early pregnancy, the placental membrane is still underdeveloped, meaning it is thick and has a small surface area.
• This makes it less permeable.

Other Hormonal Factors in Pregnancy

• Almost all the non-sexual endocrine glands in the mother's system react to pregnancy.
• This is due to the increased metabolic load on the mother and the influence of placental hormones on the endocrine glands.
• Pituitary Secretion:
  • The anterior pituitary enlarges and increases the production of Corticotropin, Thyrotropin, and Prolactin.
  • The secretion of FSH and LH is suppressed by the feedback inhibition of placental estrogen and progesterone.
• Increased Corticosteroid Secretion:
  • Pregnancy leads to increased secretion of Glucocorticoids, aiding in mobilization of amino acids from the mother's tissues for fetal tissue synthesis.
  • Also, there is an increase in Aldosterone secretion, which causes reabsorption of excess sodium leading to fluid retention.
• Increased Thyroid Gland Secretion:
  • The thyroid gland enlarges and increases its production of Thyroxine.
  • This is due to the thyrotropic effect of placental hCG and the secretion of placental human chorionic thyrotropin.
• Increased Parathyroid Gland Secretion:
  • The parathyroid gland enlarges, increasing calcium absorption from the mother's bones.
  • Maintaining calcium ion concentration in the mother's extracellular fluid while the fetus uses calcium for ossification.
• Secretion of "Relaxin" by the Ovaries and Placenta:
  • Relaxin is secreted by the corpus luteum of the ovary and the placenta.
  • This causes relaxation of the ligaments of the symphysis pubis and softens the cervix of the pregnant woman during delivery.

Response of the Mother's Body to Pregnancy

• Pregnant women experience a typical weight gain of 25 to 35 pounds.
• This includes the weight of the fetus, amniotic fluid, placenta, fetal membranes, uterus, breasts, extra fluid in the blood and ECF, and fat accumulation.

Eclampsia

• Eclampsia represents an extreme form of preeclampsia.
• It's characterized by vascular spasm throughout the body, clonic seizures in the mother (sometimes followed by coma), decreased kidney output, liver malfunction, extreme hypertension, and a generalized toxic condition of the body.
• It usually occurs shortly before childbirth.
• The best management involves vasodilating drugs followed by immediate termination of pregnancy.

Physiology of Parturition and Lactation

Parturition

• Parturition marks the birth of the baby.
• Increased Uterine Excitability Near Term:
  • There are progressive hormonal changes that cause increased excitability of the uterine musculature.
  • There are also progressive mechanical changes happening in the uterus near term.

Hormonal Factors That Increase Uterine Contractility

• Increased ratio of estrogens to progesterone:
  • During pregnancy, progesterone inhibits uterine contractility while estrogen increases it.
  • The estrogen-to-progesterone ratio increases significantly towards the end of pregnancy.
• Oxytocin:
  • Oxytocin causes uterine contractions.
  • Uterine muscle increases its oxytocin receptors in the last months of pregnancy.
  • Oxytocin secretion by the neurohypophysis is significantly increased during labor.
  • The stretching of the uterine cervix induces a hypothalamic neurogenic reflex.
• Effect of fetal hormones:
  • Fetal hormones, such as oxytocin, cortisol, and prostaglandins, also contribute to uterine contractions.

Mechanical Factors That Increase Uterine Contractility

• Stretch of the Uterine Musculature:
  • Stretching of smooth muscle organs increases their contractility.
• Stretch or Irritation of the Cervix:
  • Labor can be induced by rupturing the membranes, allowing the baby's head to stretch the cervix more forcefully.
  • Stretching of nerves in the cervix initiates reflexes to the body of the uterus, causing myogenic transmission of signals from the cervix to the body of the uterus.

Labor pains

• Cramping pain in early labor:
  • This is mainly caused by hypoxia of the uterine muscle due to the compression of blood vessels within the uterus.
  • It's initiated by the hypogastric nerves.
• Stronger pain during expulsion:
  • When the fetus is being expelled through the birth canal, pain becomes more intense due to cervical stretching, perineal stretching, and stretching or tearing of structures in the vaginal canal.
  • It's initiated by somatic nerves.

Involution of the Uterus After Parturition

• The uterus undergoes involution in the first 4 to 5 weeks after delivery.
• It reduces in weight.
• Suppression of pituitary gonadotropin and ovarian hormone secretion occurs during the first few months of lactation.
• The placental site on the endometrial surface autolyzes, resulting in a vaginal discharge known as "lochia" (initially bloody, then serous).
• The endometrial surface is re-epithelialized.

Development of the Breasts

• The glandular tissue in the breasts begins developing during puberty, but it's only during pregnancy that it fully develops and becomes capable of producing milk.
• Estrogens stimulate the growth of the breast's mammary glands and the deposition of fat, giving breasts their shape and mass.

Hormonal Regulation of Lactation

• Other hormones involved in the ductal system development are: growth hormone, prolactin, adrenal glucocorticoids, and insulin.
• Progesterone is crucial for the complete development of the lobule-alveolar system.

Prolactin and Lactation

• Prolactin, secreted by the mother's anterior pituitary gland, promotes lactation.
  • Blood prolactin levels rise steadily from the 5th week of pregnancy until childbirth.
  • Estrogen and progesterone inhibit the actual secretion of milk.
  • The placenta produces significant amounts of human chorionic somatomammotropin, which has lactogenic properties (promoting milk production).
• Colostrum is a form of milk produced during late pregnancy and the first few days after parturition.
  • It contains antibodies to protect the newborn against disease, is lower in fat, and higher in protein compared to regular milk.
  • The colostrum also has a laxative effect, helping the baby pass their first stool.

Hormonal Regulation of Lactation (part 2)

• After birth, the secretion of estrogen and progesterone from the placenta decreases, allowing prolactin's lactogenic effect to take over.
• Each time the mother nurses her baby, nervous signals from the nipples to the hypothalamus cause a surge in prolactin secretion, lasting about an hour.

Prolactin and Oxytocin

• Prolactin is responsible for the production and synthesis of milk, whereas oxytocin is involved in the release or ejection of milk.

Ovarian Cycles in Nursing Mothers

• Ovulation usually doesn't resume in nursing mothers until a few weeks after they stop breastfeeding.