lect 15 Pregnancy and Placentation: Hormones of Pregnancy

Overview of Pregnancy and Placentation

This section discusses the critical hormones and physiological adaptations that support pregnancy, specifically focusing on progesterone and its role in maintaining pregnancy, as well as the hormonal interplay between the fetus and placenta.

Maternal Support of Pregnancy

During pregnancy, the mother's body undergoes numerous changes to meet the demands of the fetus and the upcoming neonate. Some essential adaptations include:

  • Maintenance of Uterine Lining (Endometrium): The endometrium must be maintained to support implantation and growth of the embryo.
  • Uterine Musculature Development: The muscular layer of the uterus grows (hypertrophies) to allow effective expulsion of the fetus at term.
  • Mammary Gland Development: The breasts prepare for lactation to ensure post-partum nutritional support for the neonate.

These physiological adaptations are primarily regulated by hormones associated with pregnancy.

Hormones of Pregnancy

Progesterone

Progesterone is often referred to as the "hormone of pregnancy" and is crucial for the continuation of pregnancy. Key points about progesterone include:

  • It is absolutely required throughout pregnancy, and a drop in progesterone levels can lead to abortion.
  • Initially secreted by the corpus luteum (CL) after ovulation, the placenta eventually takes over progesterone production, which varies among species.

Progesterone Secretion Levels

In humans, the levels of progesterone during pregnancy differ significantly from the luteal phase in a non-pregnant state:

  • Non-pregnant luteal phase: less than 11 ng/day
  • Pregnant state: greater than 200 mg/day
  • After about 6-9 weeks of gestation, the placenta begins synthesizing progesterone from maternal cholesterol.

Androgen Production for Estrogen Synthesis

Both the human fetus and the placenta work together to produce estrogens. This process involves:

  • The utilization of androgens as substrates for estrogen production.
  • The fetal adrenal glands convert progestogens to dehydroepiandrosterone (DHA), which can also be sourced from maternal adrenal glands.
  • The fetal liver is responsible for converting progestogens to androstenedione.

Estrogen Production

The synthesis of estrogens is a collaborative effort between the fetus, placenta, and mother:

  • The placenta converts androgens into estrogens, producing:
    • Oestradiol 17β: derived from DHA
    • Oestrone: also derived from DHA
    • Oestriol: formed from 16α-hydroxylated androgens synthesized in the fetal liver.

Protection of the Fetus from High Steroid Levels

Despite the high levels of steroids produced during pregnancy, mechanisms ensure fetal safety:

  • High levels of androgen can masculinize female fetuses and pose risks.
  • All steroids transferred from mother/placenta to the fetus are conjugated in the fetal liver and adrenal glands to become biologically inactive sulfonated derivatives.
  • These conjugated steroids are then released into the mother's circulation as biologically active hormones by the placenta, which de-sulfates them, enabling high maternal steroid levels while keeping fetal levels low and inactive.

Summary of Hormonal Dynamics During Pregnancy

In summary, the hormonal interactions and syntheses are as follows:

  • Placenta synthesizes progesterone.
  • Fetus produces C19-androgens from progestagens.
  • Placenta synthesizes estrogens from C19-androgens.
  • Fetus conjugates steroids to sulfate forms, rendering them biologically inactive. The placenta then converts these to active hormones, maintaining a balance beneficial for both mother and fetus.