Progesterone- The Pregnancy Hormone

 

Progesterone- The Pregnancy Hormone

Introduction

Progesterone is one of the principal female sex steroid hormones and plays a central role in reproductive physiology, pregnancy maintenance, menstrual cycle regulation, and mammary gland development. Along with estrogen, progesterone coordinates the cyclic changes in the female reproductive system.

The term progesterone is derived from the phrase “pro-gestation hormone”, reflecting its critical role in preparing the uterus for implantation and maintaining pregnancy.

Progesterone is primarily secreted by:

• Corpus luteum of the ovary
• Placenta during pregnancy
• Adrenal cortex (small amounts)

In males, small quantities are also produced by:

• Testes
• Adrenal glands

Progesterone belongs to the class of C-21 steroid hormones synthesized from cholesterol.

Learning Objectives

After studying this topic, the student should be able to:

1. Describe the chemistry and synthesis of progesterone.
2. Explain regulation of progesterone secretion.
3. Discuss physiological actions of progesterone.
4. Explain progesterone’s role in the menstrual cycle and pregnancy.
5. Describe transport, metabolism, and mechanism of action.
6. Discuss clinical importance and disorders related to progesterone.

Chemistry and Structure

Progesterone is:

• A steroid hormone
• Derived from cholesterol
• Lipid soluble
• Molecular formula: C₂₁H₃₀O₂

It belongs to the progestogens, a group of hormones with progesterone-like activity.

Structural Features

• 21-carbon steroid nucleus
• Ketone groups at C3 and C20
• Double bond between C4 and C5

Biosynthesis of Progesterone

Progesterone synthesis occurs in steroidogenic tissues.

Sites of Synthesis

In Females

• Corpus luteum (major source in luteal phase)
• Placenta (during pregnancy)

In Both Sexes

• Adrenal cortex

In Males

• Leydig cells of testes (minor amount)

Steroidogenesis Pathway

Progesterone is synthesized from cholesterol.

Pathway

Cholesterol→Pregnenolone→Progesterone

Important Enzymes

1. Cholesterol Desmolase (P450scc)

Converts cholesterol → pregnenolone.

2. 3β-Hydroxysteroid Dehydrogenase

Converts pregnenolone → progesterone.

Regulation of Progesterone Secretion

Role of LH

• Luteinizing hormone (LH) stimulates corpus luteum formation.
• Corpus luteum secretes progesterone after ovulation.

During Menstrual Cycle

Follicular Phase

• Minimal progesterone secretion

Luteal Phase

• Marked increase due to corpus luteum activity

During Pregnancy

Early Pregnancy

• Corpus luteum maintained by:
• Human chorionic gonadotropin (hCG)

Later Pregnancy

• Placenta becomes the major source.

Plasma Levels During Menstrual Cycle

Follicular Phase

• Very low

After Ovulation

• Rapid increase

Mid-Luteal Phase

• Peak secretion

Before Menstruation

• Falls sharply if pregnancy does not occur

This fall triggers menstruation.

Transport in Blood

Progesterone circulates mainly bound to plasma proteins:

• Albumin
• Corticosteroid-binding globulin (CBG)

Only a small fraction remains free and biologically active.

Mechanism of Action

Being lipid soluble, progesterone acts through intracellular nuclear receptors.

Steps

1. Diffuses through cell membrane
2. Binds intracellular progesterone receptor
3. Hormone-receptor complex enters nucleus
4. Binds hormone response elements on DNA
5. Alters gene transcription
6. Protein synthesis changes

Progesterone Receptors

Two main receptor isoforms:

• PR-A
• PR-B

These receptors belong to the nuclear receptor superfamily.

Physiological Actions of Progesterone

1. Actions on Uterus

This is the most important action.

Endometrial Changes

Progesterone converts proliferative endometrium into secretory endometrium.

Effects

• Increased glandular secretion
• Increased vascularity
• Endometrial edema
• Glycogen accumulation

These changes prepare the uterus for implantation.

Secretory Transformation

Under estrogen influence:

• Endometrium proliferates

Under progesterone influence:

• Endometrium becomes secretory and receptive.

Decreased Uterine Contractility

Progesterone:

• Reduces excitability of uterine smooth muscle
• Reduces sensitivity to oxytocin

This helps maintain pregnancy.

2. Actions on Cervix

Progesterone causes:

• Thick, viscid cervical mucus
• Reduced sperm penetration

This contrasts with estrogen, which produces thin watery mucus.

3. Actions on Fallopian Tubes

Progesterone:

• Increases secretions in tubes
• Provides nutrition for fertilized ovum

4. Actions on Vagina

Progesterone:

• Promotes desquamation of vaginal epithelium
• Opposes estrogenic proliferation

5. Actions on Breasts

Progesterone promotes:

• Development of lobules and alveoli
• Preparation for lactation

However, high progesterone levels inhibit actual milk secretion during pregnancy.

6. Thermogenic Action

Progesterone increases basal body temperature by about: 0.3∘C to 0.5∘C

This occurs after ovulation and forms the basis of:

• Basal body temperature method for ovulation detection

7. Respiratory Effects

Progesterone stimulates respiratory center.

Effects:

• Mild hyperventilation
• Decreased arterial PCO2

Important during pregnancy.

8. Metabolic Effects

Protein Metabolism

• Mild anabolic effect

Fat Metabolism

• Promotes fat deposition

Carbohydrate Metabolism

• Mild increase in insulin secretion

9. Renal Effects

Progesterone acts as a weak antagonist to aldosterone.

Effects:

• Mild natriuresis
• Sodium loss

10. CNS Effects

Progesterone has:

• Sedative effects
• Calming action

Some metabolites act on GABA-A receptors.

Role in Menstrual Cycle

Progesterone dominates the luteal phase.

Phases of Menstrual Cycle

Follicular Phase

• Estrogen dominant

Ovulation

• LH surge

Luteal Phase

• Progesterone dominant

Endometrial Cycle

Proliferative Phase

Stimulated by estrogen.

Secretory Phase

Stimulated by progesterone.

Withdrawal of Progesterone

If fertilization does not occur:

• Corpus luteum degenerates
• Progesterone falls
• Spiral arteries constrict
• Endometrial shedding occurs

→ Menstruation

Progesterone and Pregnancy

Progesterone is essential for successful pregnancy.

Functions During Pregnancy

1. Maintains Endometrium

Keeps decidua healthy.

2. Prevents Uterine Contractions

Helps prevent abortion.

3. Immune Modulation

Helps maternal tolerance of fetus.

4. Breast Preparation

Prepares mammary glands for lactation.

5. Cervical Integrity

Maintains cervical closure.

Placental Progesterone Production

By approximately 8–10 weeks:

• Placenta becomes primary source.

This is called:

• Luteo-placental shift

Progesterone Withdrawal and Labor

Functional progesterone withdrawal contributes to:

• Initiation of labor
• Increased uterine sensitivity to oxytocin

Progesterone in Males

Small amounts are produced in males.

Functions:

• Precursor for testosterone synthesis
• Precursor for corticosteroids

Pharmacological Preparations

Natural Progesterone

Micronized progesterone

Synthetic Progestins

Examples:

• Medroxyprogesterone
• Norethindrone
• Levonorgestrel

Clinical Applications

1. Hormonal Contraception

Used in:

• Oral contraceptive pills
• Injectable contraceptives
• Implants
• IUCD systems

Mechanisms:

• Suppresses ovulation
• Thickens cervical mucus
• Makes endometrium unsuitable

2. Hormone Replacement Therapy

Combined with estrogen to:

• Prevent endometrial hyperplasia

3. Infertility Treatment

Used in:

• Luteal phase support
• Assisted reproductive techniques

4. Prevention of Preterm Labor

Progesterone supplementation may reduce risk in selected women.

5. Dysfunctional Uterine Bleeding

Helps regulate endometrium.

Disorders Related to Progesterone

1. Luteal Phase Defect

Inadequate progesterone secretion causes:

• Infertility
• Early pregnancy loss

2. Progesterone Deficiency

Can result in:

• Irregular menstruation
• Failure of implantation
• Recurrent miscarriage

3. Premenstrual Syndrome (PMS)

Fluctuating progesterone levels may contribute.

4. Congenital Adrenal Hyperplasia

Progesterone intermediates accumulate due to enzyme defects.

Laboratory Assessment

Serum Progesterone Levels

Used to:

• Confirm ovulation
• Assess luteal function
• Evaluate pregnancy

Mid-Luteal Progesterone

Indicates ovulation occurred.

Progesterone and Ovulation Testing

Rise in progesterone:

• Indicates corpus luteum formation
• Confirms ovulation

Comparison Between Estrogen and Progesterone

Feature	Estrogen	Progesterone
Main phase	Follicular	Luteal
Endometrium	Proliferative	Secretory
Uterine muscle	Increases excitability	Decreases excitability
Cervical mucus	Thin, watery	Thick, viscid
Basal body temperature	No rise	Raises temperature
Breast effect	Ductal growth	Lobuloalveolar growth

High-Yield Facts

• Progesterone is secreted mainly by corpus luteum.
• It converts proliferative endometrium into secretory endometrium.
• It decreases uterine contractility.
• Progesterone raises basal body temperature.
• Placenta becomes major source during pregnancy.
• Withdrawal of progesterone contributes to menstruation.

Important Examination Points

Frequently Asked Short Notes

• Physiological actions of progesterone
• Role of progesterone in pregnancy
• Mechanism of action of steroid hormones
• Progesterone during menstrual cycle
• Corpus luteum and progesterone secretion

Important Viva Questions

1. What is the source of progesterone after ovulation?
2. Which hormone maintains corpus luteum in early pregnancy?
3. Why does basal body temperature rise after ovulation?
4. What is the effect of progesterone on cervical mucus?
5. What is luteo-placental shift?
6. Which phase of menstrual cycle is progesterone dominant?

Summary

Progesterone is an essential steroid hormone in female reproductive physiology. Secreted mainly by the corpus luteum and placenta, it prepares the uterus for implantation, maintains pregnancy, suppresses uterine contractions, and supports mammary gland development. Its cyclic secretion coordinates with estrogen to regulate the menstrual cycle. Understanding progesterone physiology is fundamental for clinical medicine, gynecology, endocrinology, infertility management, and obstetrics.

Suggested Standard References

Textbooks

1. Guyton and Hall Textbook of Medical Physiology
2. Ganong's Review of Medical Physiology
3. Vander's Human Physiology
4. Williams Gynecology
5. Williams Obstetrics

Recommended Guidelines

• World Health Organization
• American College of Obstetricians and Gynecologists