The first thing an implanted embryo does is send a chemical message telling its mother’s body not to have a period. Everything else about pregnancy follows from keeping that lining in place — right up until the moment the foetus signals that it is time to leave.
📚 What you need to know
Shortly after implanting, the embryo secretes human chorionic gonadotropin (hCG), for roughly the first 8–10 weeks.
hCG stimulates the corpus luteum to keep secreting oestrogen and progesterone, stimulates the growth of the placenta and uterine enlargement, and inhibits menstruation.
After about 12 weeks hCG declines and the placenta takes over secreting oestrogen and progesterone.
Pregnancy tests detect hCG in urine using monoclonal antibodies.
Progesterone inhibits oxytocin production and inhibits contractions of the myometrium throughout pregnancy.
At the end of pregnancy the foetus produces oestrogen, which signals the placenta to stop producing progesterone and makes the uterine wall more sensitive to oxytocin.
Oxytocin stimulates contractions, and stretch receptors in the cervix drive a positive feedback loop that increases them.
Human chorionic gonadotropin
Think about the problem the embryo faces. It has just implanted into an endometrium that, on a normal cycle, would be shed in a few days when the corpus luteum breaks down. If that happened, the embryo would go with it.
So the embryo takes control. Shortly after implantation it begins secreting hCG, which stimulates the corpus luteum in the ovary to maintain its secretion of oestrogen and progesterone. Those two hormones continue the development of the endometrium, and menstruation is inhibited. hCG also stimulates the growth of the placenta and enlargement of the uterus.
This is a temporary arrangement. During the second trimester, after about 12 weeks, hCG declines and the placenta takes over the role of secreting oestrogen and progesterone directly. The corpus luteum is no longer needed.
The hCG curve is the giveaway in any exam graph: it is the only one that peaks early and then falls away.
Pregnancy tests
Because hCG appears in the blood and urine within days of implantation and is present in no other situation, it is a near-perfect marker for pregnancy.
A pregnancy test kit uses a monoclonal antibody (mAb) — a single type of antibody that works outside the body and reacts with one specific antigen, in this case hCG. Urine travels along the test strip; if hCG is present, the mobile monoclonal antibodies bind to it, and the complex is trapped by a layer of fixed antibodies in the first window, producing a visible colour change. If there is no colour change, hCG is not present in high enough concentrations to bind.
The second window is the control. Antibodies that have not bound hCG move on and are trapped there, producing a line that shows the antibodies have moved up the strip properly — which is what tells you that a blank first window really means a negative result rather than a broken test.
Keeping the pregnancy going
Progesterone is secreted by the placenta throughout pregnancy, and it has two jobs beyond maintaining the endometrium. It inhibits the production of oxytocin by the pituitary gland, and it inhibits contractions of the muscles of the uterus wall, the myometrium, which could otherwise induce birth early.
Starting labour
At the end of pregnancy the foetus produces oestrogen. This signals to the placenta to stop producing progesterone, and with progesterone gone the brake comes off: the pituitary gland begins producing oxytocin. Oestrogen also makes the uterine wall more sensitive to oxytocin, and inhibits progesterone directly.
Oxytocin, released by the pituitary gland, stimulates contractions of the myometrium. Stretch receptors in the cervix detect the contractions and signal the pituitary gland to increase oxytocin secretion. More oxytocin causes stronger contractions, which stretch the cervix further, which signals for still more oxytocin. This is positive feedback, and it increases the contractions slowly and rhythmically until birth.
This is the standard example of positive feedback in the whole syllabus, precisely because it has a clear end point: the baby is born and the stretching stops.
Birth and afterbirth
Relaxation of the cervix muscles causes the cervix to dilate and widen. Uterine contractions continue and cause the amniotic sac to burst, releasing the amniotic fluid through the open cervix. The baby is then pushed out through the cervix and vagina as contractions continue. When the umbilical cord is cut, the baby is physiologically separated from the mother.
After delivery, uterine contractions continue and the placenta separates from the uterine wall. The mother then births the placenta and the remains of the umbilical cord — the afterbirth.
Worked examples
WORKED EXAMPLE
Explain why hCG levels fall after about 12 weeks without the pregnancy ending. [3]
Point 1 — what hCG was for
hCG stimulates the corpus luteum to keep secreting oestrogen and progesterone, which maintain the endometrium.
Point 2 — the handover
By the second trimester the placenta has grown enough to secrete oestrogen and progesterone itself.
Point 3 — the consequence
The corpus luteum is no longer needed, so hCG declines while the endometrium continues to be maintained by placental hormones.
3 markshCG stimulated placental growth in the first place, so it effectively built its own replacement.
WORKED EXAMPLE
Explain the role of progesterone in preventing birth before full term. [3]
Point 1 — on the muscle
Progesterone inhibits contractions of the myometrium, the muscle of the uterus wall.
Point 2 — on the hormone
It also inhibits production of oxytocin by the pituitary gland, so the signal that would trigger contractions is suppressed.
Point 3 — the end of pregnancy
At full term, oestrogen from the foetus signals the placenta to stop producing progesterone, removing both inhibitions and allowing labour to begin.
3 marksProgesterone is the brake, oxytocin is the accelerator. Say which is which.
💡 Exam tip
Learn the three hCG roles: maintain the corpus luteum, grow the placenta, inhibit menstruation.
On a pregnancy hormone graph, identify hCG by its early peak and decline.
Say myometrium when discussing uterine contractions.
Label the labour loop explicitly as positive feedback, and say what ends it.
For pregnancy tests, use the terms monoclonal antibody, specific antigen and colour change.
The trigger for labour comes from the foetus, producing oestrogen. That surprises people.
⚠ Common mix-up
hCG is not made by the mother’s pituitary. The embryo, and later the placenta, secrete it.
Falling hCG does not mean the pregnancy is failing. It means the placenta has taken over.
Oxytocin is not progesterone’s partner. They oppose each other.
Positive feedback is not “good” feedback. It means the response amplifies the stimulus.
The corpus luteum does not last the whole pregnancy. It is only needed for the first trimester.
A pregnancy test detects hormone, not the embryo. It is measuring hCG in urine.
Up next: The Placenta — the organ that took over from the corpus luteum, and how it feeds a foetus without the two blood supplies ever touching.
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