| First detectable hCG | As early as 10–14 days after fertilisation (American College of Obstetricians and Gynecologists (ACOG)) |
| Peak hCG level | Typically weeks 8–10 of pregnancy (ACOG clinical guidance) |
| Placental progesterone takeover | Around 8–10 weeks gestation (Endocrine Society) |
| Oestrogen rise by third trimester | Up to 1,000-fold above pre-pregnancy baseline (Endocrine Society, reproductive endocrinology literature) |
| Relaxin peak timing | First trimester, then again near term (Journal of Physiology research literature) |
| Prolactin level at term | Approximately 10–20× non-pregnant baseline (Endocrine Society) |
First Trimester: The hCG Surge and Hormonal Foundation
The moment a fertilised egg implants in the uterine lining, your body begins a remarkable hormonal cascade. The developing placenta starts producing human chorionic gonadotropin (hCG), which signals the ovaries to keep releasing progesterone — preventing menstruation and preserving the pregnancy before the placenta can take over that role itself.
| First detectable hCG | As early as 10–14 days after fertilisation (American College of Obstetricians and Gynecologists (ACOG)) |
| Peak hCG level | Typically weeks 8–10 of pregnancy (ACOG clinical guidance) |
| Placental progesterone takeover | Around 8–10 weeks gestation (Endocrine Society) |
| Oestrogen rise by third trimester | Up to 1,000-fold above pre-pregnancy baseline (Endocrine Society, reproductive endocrinology literature) |
| Relaxin peak timing | First trimester, then again near term (Journal of Physiology research literature) |
| Prolactin level at term | Approximately 10–20× non-pregnant baseline (Endocrine Society) |
hCG roughly doubles every 48–72 hours in a healthy early pregnancy, peaking between weeks 8 and 10 before declining to a lower, stable level. This surge is directly linked to nausea and vomiting of pregnancy (commonly called morning sickness), though the precise mechanism remains an active area of research.
Progesterone rises sharply, relaxing smooth muscle throughout the body — which explains slower digestion, heartburn, and that pervasive first-trimester fatigue. Oestrogen begins climbing too, stimulating uterine growth and increasing blood volume. Relaxin is also elevated, beginning to loosen pelvic ligaments — sometimes contributing to hip or lower-back discomfort even this early.
For a broader look at how these hormones fit into a woman's overall health story, see our reference on hormonal health across the lifespan.
Second Trimester: The Placenta Takes Charge
By the start of the second trimester, the placenta has fully assumed hormone production — a transition called the luteal-placental shift. Progesterone and oestrogen continue rising steadily, but hCG has tapered off. For many women, this is when nausea eases and energy returns.
Oestrogen (primarily oestriol, produced by the foetal-placental unit) supports foetal brain, lung, and organ development. It also increases skin blood flow — the physiological basis for that often-described pregnancy 'glow,' though it can equally trigger skin changes like melasma.
Prolactin, produced by the pituitary gland, climbs steadily through the second trimester, quietly priming breast tissue for eventual milk production. Breast tenderness may persist or shift in character as this process continues.
Human Chorionic Gonadotropin (hCG)
A hormone produced by the developing placenta shortly after fertilisation. It maintains progesterone production in early pregnancy and is the hormone detected by home pregnancy tests.
Progesterone
A steroid hormone essential for maintaining the uterine lining and suppressing uterine contractions during pregnancy. It rises dramatically in the first trimester and is produced mainly by the placenta after about 10 weeks.
Oestrogen (Estradiol/Estriol)
A group of hormones that peak during the third trimester, supporting foetal organ development, uterine growth, and preparation of breast tissue for milk production.
Relaxin
A peptide hormone that loosens ligaments and softens the cervix, allowing the pelvis to widen in preparation for delivery. Elevated levels can contribute to joint instability.
Oxytocin
Often called the 'bonding hormone,' oxytocin triggers uterine contractions during labour and stimulates the milk let-down reflex during breastfeeding.
Prolactin
A pituitary hormone that rises steadily throughout pregnancy, priming the breasts for milk production after birth.
Progesterone's muscle-relaxing properties, helpful for preventing premature contractions, also keep causing gastrointestinal symptoms including constipation. Staying well-hydrated and eating fibre-rich foods are commonly recommended supportive steps — though discuss any persistent symptoms with your prenatal provider.
Third Trimester: Preparing for Birth and Beyond
The final trimester is defined by an oestrogen surge of remarkable scale — levels can reach roughly 1,000 times pre-pregnancy baseline. This hormonal peak serves critical functions: ripening the cervix, increasing the uterus's sensitivity to oxytocin, and accelerating foetal lung maturation.
~1,000×
Rise in oestrogen levels by late pregnancy
Oestriol, produced largely by the foetal-placental unit, increases dramatically across the third trimester according to reproductive endocrinology literature reviewed by the Endocrine Society.
10–20×
Increase in prolactin at term vs. baseline
Prolactin levels rise steadily from the first trimester onward, reaching 10–20 times non-pregnant levels by the time of delivery, per Endocrine Society guidance.
Oxytocin receptors in the uterus proliferate in response to rising oestrogen, setting the stage for labour. When labour begins, oxytocin release follows a positive feedback loop — contractions stimulate more oxytocin, which drives stronger contractions, until delivery occurs.
Cortisol — the body's primary stress hormone — also rises through the third trimester, contributing to immune modulation and metabolic adjustments. Elevated cortisol is physiologically normal during this period, though chronic psychosocial stress can complicate this picture. The connection between hormones and mood is explored in depth in our piece on the hormonal roots of depression in women.
Hormonal Shifts and Mood: A Real Connection
The rapid hormonal changes of pregnancy — particularly progesterone's sedating effects and oestrogen's influence on serotonin — can significantly affect emotional wellbeing. If you're experiencing persistent low mood, anxiety, or feel overwhelmed at any point in pregnancy, speaking with your midwife, OB, or a mental health professional is a meaningful step. See our overview of perinatal mental health for a fuller picture.
After delivery, oestrogen and progesterone drop sharply — a shift that has real consequences for mood, energy, and physical recovery. Our article on the postpartum hormonal shift explains what to expect in those early weeks.
This article is for general informational purposes only and does not constitute medical advice. Always consult a qualified healthcare professional for guidance specific to your pregnancy and health needs.