hCG and the corpus luteum in early pregnancy
hCG (human chorionic gonadotrophin), a placental hormone, rescues the corpus luteum, a temporary structure in the ovary that makes progesterone, in early pregnancy.
Remember
In early pregnancy, hCG rescues the corpus luteum, so progesterone keeps being made until the placenta can take over.
After ovulation, the corpus luteum (a temporary structure in the ovary that secretes progesterone) keeps the womb lining ready for a pregnancy. Without a pregnancy it shrinks after about two weeks, and the lining comes away as a period.
If an embryo implants, the syncytiotrophoblast (the outer cell layer of the developing placenta) releases human chorionic gonadotrophin (hCG), a protein hormone. In a non-pregnant cycle, luteinising hormone (LH) is the signal that sustains the corpus luteum. In pregnancy, hCG acts like LH, so it rescues the corpus luteum and progesterone production continues.
During the first trimester (the early part of pregnancy) the corpus luteum is therefore the main source of progesterone. The placenta is not yet mature enough to make enough on its own. At about the start of the second trimester it makes enough oestrogen and progesterone itself, and the corpus luteum is no longer needed. This hand-over is sometimes called the luteoplacental shift.
In the first trimester, if hCG falls, the corpus luteum regresses (shrinks and stops working), progesterone support fails and the pregnancy is at risk of loss. Because hCG is made only by the syncytiotrophoblast, a tissue that exists only in pregnancy, it is specific to pregnancy: it is not normally produced in any other situation, so urine and blood pregnancy tests detect it.
hCG is the stand-in for LH: in pregnancy it sustains the corpus luteum, the job that LH does in the cycle.
Key facts
- Source
- hCG comes from the syncytiotrophoblast, from very early in pregnancy.
- Job
- it rescues the corpus luteum, so progesterone production continues through the first trimester.
- Hand-over
- the placenta makes its own oestrogen and progesterone from about the start of the second trimester.
- LH link
- hCG acts like luteinising hormone, which sustains the corpus luteum in a non-pregnant cycle.
- Pregnancy tests
- all pregnancy tests detect hCG, in urine or in blood.
Exam traps
- Progesterone is mainly made by the corpus luteum in early pregnancy, so it is what the corpus luteum delivers, not the signal that maintains it; hCG is that signal.
- Luteinising hormone sustains the corpus luteum in a cycle without pregnancy, but in pregnancy hCG takes over that job.
- The placenta makes enough progesterone itself only once it is mature, around the start of the second trimester, so placental progesterone is not the signal that keeps the corpus luteum going.
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