Placental hormones estrogen and progestrone chnages during pregancy

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Placental Hormones: Estrogen and Progesterone Changes During Pregnancy

Overview

During pregnancy, estrogen and progesterone are produced first by the corpus luteum, then progressively taken over by the placenta - a shift called the luteo-placental shift. Both hormones rise continuously throughout gestation and perform distinct, essential roles.

Progesterone

Source and the Luteo-Placental Shift

Weeks 1-10 (first trimester): Progesterone is produced exclusively by the corpus luteum of pregnancy, which is rescued from degeneration by hCG (human chorionic gonadotropin) secreted by syncytiotrophoblasts. The corpus luteum remains functionally active through the first ~20 weeks.
~End of first trimester onward: The placenta takes over as the principal source of progesterone, synthesizing it de novo from maternal cholesterol. This is the luteo-placental shift.
Placental progesterone synthesis markedly increases with advancing gestation, producing very high maternal circulating levels that persist until delivery of the placenta.
- Creasy & Resnik's Maternal-Fetal Medicine, p. 148
- The Developing Human (Clinically Oriented Embryology), p. 104

Key Levels

Progesterone levels are low before pregnancy (~1-3 ng/mL in the follicular phase), rise in the luteal phase (~10-35 ng/mL), and continue climbing throughout pregnancy, reaching ~100-200 ng/mL or higher at term.

Actions During Pregnancy

Action	Mechanism
Uterine quiescence	Acts on uterine smooth muscle via nuclear progesterone receptors (PR-A and PR-B) to block contractions
Inhibits prostaglandin production	Delays cervical ripening and labor
Endometrial maintenance	Supports decidua; prevents menstruation
Immunomodulation	Inhibits T-lymphocyte-mediated tissue rejection of the fetal allograft
Mammary development	Stimulates alveolar growth in breast tissue
Cervical closure	Promotes PR-mediated gene transcription that keeps the cervix closed and rigid

Progesterone and Parturition

Uniquely in humans, systemic progesterone levels do not fall before labor (unlike other mammals). Instead, a "functional progesterone withdrawal" occurs - uterine cells become progressively refractory to progesterone signaling via:

Increased expression of PR-A (which represses PR-B activity) in myometrial cells
Phosphorylation of PR-A driven by proinflammatory stimuli
Decreased PR coactivators
- Creasy & Resnik's Maternal-Fetal Medicine, pp. 148-149

Estrogen

Source: The Fetoplacental Unit

A key feature of estrogen in pregnancy is that the placenta cannot make estrogen alone - it lacks 17α-hydroxylase, so it cannot convert C-21 precursors to the C-19 androgens needed for estrogen synthesis. Estrogen production therefore depends on a fetoplacental cooperation:

Fetal adrenal glands produce large amounts of DHEA-S (dehydroepiandrosterone sulfate)
DHEA-S is transported to the placenta, which aromatizes it to estrogens
For estriol (E3) specifically, fetal liver first 16α-hydroxylates the DHEA-S before placental aromatization

This is why estriol levels reflect fetal wellbeing - if the fetus or its adrenals are compromised (e.g., anencephaly, fetal demise), estriol levels fall dramatically.

- Tietz Textbook of Laboratory Medicine (7th ed.), p. 2406
- Creasy & Resnik's Maternal-Fetal Medicine

The Three Estrogens in Pregnancy

Estrogen	Abbreviation	Role/Notes
Estrone	E1	Minor contributor
Estradiol	E2	Primary ovarian estrogen; rises in pregnancy
Estriol	E3	The dominant estrogen of pregnancy; requires fetal precursors

Quantitative Changes

Non-pregnant: Ovaries secrete 100-600 μg/day of estradiol; ~10% is metabolized to estriol.
Late pregnancy (term): The placenta produces:
- Estriol: 50-150 mg/day (100-1000x increase)
- Estradiol + Estrone: 15-20 mg/day each
- Tietz Textbook of Laboratory Medicine (7th ed.), p. 2406

Actions of Estrogen in Pregnancy

Action	Effect
Uterine growth	Promotes hypertrophy and hyperplasia of myometrium
Uteroplacental blood flow	Increases uterine vascular supply
Cervical ripening	Softening and effacement prior to labor
Breast development	Ductal proliferation (with progesterone)
Upregulates oxytocin & PG receptors	Prepares uterus for labor contractions
Sodium retention	Contributes to plasma volume expansion of pregnancy

Estrogen and progesterone work together to ensure (1) appropriate endometrial development, (2) uterine growth, (3) adequate uterine blood supply, and (4) preparation of the uterus for labor - Tietz Textbook of Laboratory Medicine, p. 2406.

Summary: Hormonal Trajectory Through Pregnancy

          Estrogen (esp. Estriol)        Progesterone
Week 1-6:  ↑ (corpus luteum)             ↑ (corpus luteum)
Week 6-10: ↑ (CL + early trophoblast)   ↑ (corpus luteum dominant)
Week 10+:  ↑↑ (luteo-placental shift)   ↑↑ (placenta takes over)
Week 20+:  ↑↑↑ (fetoplacental unit)     ↑↑↑ (placenta sole source)
Term:      Massive ↑ (50-150 mg/day E3) Very high; no systemic fall
Labor:     Estrogen/P ratio ↑           Functional withdrawal (receptor level)
Post-delivery: Rapid ↓ (placenta gone)  Rapid ↓ (placenta gone)

Clinical Pearls

Estriol in maternal serum screening: Second-trimester unconjugated estriol is a component of the "triple/quad screen" - low levels suggest fetal trisomy 21 or 18.
Progesterone supplementation: Used clinically (vaginal or IM 17-hydroxyprogesterone caproate) to prevent preterm birth in at-risk women, exploiting progesterone's uterine quiescence properties.
Aromatase deficiency: If the placenta or fetus lacks aromatase, maternal and fetal virilization occurs because androgens cannot be converted to estrogen - confirming the fetoplacental unit's role.
Molar pregnancy / fetal demise: Estrogen levels are low because the fetal contribution to the fetoplacental unit is absent.

Sources: Tietz Textbook of Laboratory Medicine 7th ed. | Creasy & Resnik's Maternal-Fetal Medicine | The Developing Human (Moore) | Medical Physiology (Boron & Boulpaep) | Goodman & Gilman's Pharmacology

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