Which hormone is primarily responsible for maintaining pregnancy in the early weeks until placenta takes over?

Appeared in: RML PRE 2025

Explanation

Progesterone is the primary hormone responsible for maintaining pregnancy in the early weeks.
It is initially secreted by the corpus luteum, which is sustained by human chorionic gonadotropin (hCG).
Progesterone's main functions are to maintain the decidual lining of the uterus, providing nutrition for the embryo, and to inhibit myometrial contractions, preventing spontaneous abortion.
Around 8-12 weeks, the placenta develops sufficiently to take over progesterone production, a critical event known as the luteal-placental shift.

Option A: While estrogen is crucial for pregnancy, its primary roles are promoting the growth of the uterus, breasts, and external genitalia, not directly maintaining uterine quiescence in the same way as progesterone.
Option B: Prolactin's main function is related to lactation (milk production). While its levels rise during pregnancy, it does not play a primary role in maintaining the uterine environment for the embryo.
Option C: Luteinizing hormone (LH) is essential for triggering ovulation and the initial formation of the corpus luteum. However, once pregnancy is established, hCG takes over the function of stimulating the corpus luteum, and LH levels are suppressed.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Hormonal regulation of early pregnancy as background academic context rather than a clinical decision trigger.
Understanding progesterone's role is critical for managing threatened miscarriages. Progesterone supplements are sometimes prescribed to support the uterine lining in early pregnancy if a deficiency is suspected (luteal phase defect).
In assisted reproductive technologies (ART) like IVF, progesterone support is a standard part of the protocol to ensure the endometrium is receptive to the embryo and to maintain the pregnancy after transfer.
What if? If a pregnant patient's hCG levels are not doubling appropriately in the first few weeks, what is the immediate concern regarding progesterone? A slow rise in hCG indicates poor trophoblastic function, which means the corpus luteum won't be adequately stimulated. This leads to insufficient progesterone production, endangering the pregnancy and increasing the risk of spontaneous abortion.

How to Approach the Question

First, identify the core concept of the question: which hormone is essential for maintaining the very early stages of pregnancy.
Recall the sequence of events after fertilization: the role of the corpus luteum is paramount before the placenta is fully functional.
Evaluate each hormone's primary function in the context of reproduction.
Progesterone is known as the 'hormone of pregnancy' precisely because it maintains the uterine lining and prevents contractions. This makes it the strongest candidate.
Eliminate the other options based on their primary roles: Estrogen (growth), Prolactin (lactation), and LH (ovulation trigger, superseded by hCG in pregnancy).
Confirm that Progesterone's role fits the timeline specified ('early weeks until placenta takes over').

Concept Tested & Keywords

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E6 Physiology Guyton 4SAE Part 3 p. 60-62

E6 Medicine Harrison 22e Part 2 p. 1032-1034

E6 Obstetrics Williams p. 2-9

Practise the full RML PRE 2025

Attempt every question from this paper in a timed mock, then review the full solution for each one.