--- title: 'What Hormonal Changes Happen During The First Trimester?' slug: hormonal-changes-early-pregnancy-first-trimester lifeStage: pregnancy category: Cycle author: 'Samaira Farid' medicalReviewer: 'Dr Shree Datta, Consultant Obstetrician and Gynaecologist ' reviewerId: 'https://blog.bloume.com/pregnancy/medical-board.md#dr-shree-datta' lastUpdated: '2026-08-29' primarySource: 'Physiological Changes in Pregnant Women Due to Hormonal Changes' wordCount: 1082 readTimeInSeconds: 271 --- # What Hormonal Changes Happen During The First Trimester? The first trimester brings a fast shift in your hormones that drives almost every physical change you notice. hCG and progesterone surge within days of implantation, bringing fatigue, nausea, and digestive changes. Oestrogen rises steadily alongside them, supporting your baby's development and the growth of your uterus. By the end of the first trimester your hormone levels start to settle and many symptoms ease, though your own experience may differ. This article explains what is happening and why. **Q: What are the main hormonal changes in early pregnancy during the first trimester?** A: Three hormones drive early pregnancy: human chorionic gonadotropin (hCG), progesterone, and oestrogen. Your developing placenta makes hCG soon after implantation, and it is the hormone a pregnancy test picks up. It rises fast in the first weeks, signalling your ovaries to keep making progesterone. Progesterone climbs sharply to hold the uterine lining in place, support the growing placenta, and prevent early contractions. Oestrogen rises steadily alongside them, supporting your baby's organs and the growth of your uterus. By ten to twelve weeks, the placenta takes over progesterone and levels start to settle. Early pregnancy brings some of the most significant hormonal changes your body will go through, and they happen fast. The exhaustion, nausea, and emotional shifts that arrive in the first trimester are all a direct response to that shift. Understanding what is driving them can make the physical side easier to read. ## The hormone that starts it all: hCG Within days of the embryo settling into the wall of your uterus, the developing placenta starts producing a hormone called hCG. It is what a pregnancy test picks up, and its presence is the first sign that a pregnancy has started. In the earliest weeks, hCG rises fast, roughly doubling every two to three days. Its main job right now is to tell your ovaries to keep making progesterone, the hormone that holds the pregnancy steady. This quick rise is normal, though the exact pace differs from person to person. ## Progesterone: sustaining the pregnancy through the first trimester Progesterone climbs sharply after conception. It keeps the lining of your uterus in place, supports the growing placenta, and stops the muscles of your womb contracting too early. In the first weeks your ovaries make most of it, then the placenta gradually takes over and becomes the main source by around ten to twelve weeks. This handover is a normal part of how the first trimester moves along. You may notice some symptoms ease around this point as your hormones settle, though that does not happen for everyone. If symptoms carry on past twelve weeks, that is within the normal range. ## Oestrogen's role alongside progesterone Oestrogen rises steadily across the first trimester, working alongside hCG and progesterone. It supports your baby's organs as they develop and helps your uterus grow to make room. Much of what you feel physically in these early weeks comes down to these three hormones rising together. ## Why fatigue and nausea arrive so early in the first trimester The bone-deep tiredness that can arrive in the first weeks ties closely to the sharp rise in progesterone. Progesterone has a sleep-inducing effect, so you can feel wiped out even after a full night's sleep. This is your body responding to a real physical shift. The nausea and vomiting often called morning sickness is mostly tied to the fast rise in hCG. It tends to peak around weeks nine to ten, when hCG is at its highest. Despite the name, it can hit at any time of day. > **WARNING**: Morning sickness is common and usually settles with time. For a small number of people it becomes more severe, a condition called hyperemesis gravidarum (HG), which brings constant vomiting and trouble keeping fluids down. If you cannot keep fluids or food down, or nausea is taking over your day, speak to your midwife or GP. ## Other physical effects of the hormonal surge The bloating, wind, and constipation that can show up early come down to progesterone too. It relaxes smooth muscle across your body, including in your digestive tract, which slows everything down. Keeping your fluids up and eating fibre-rich foods can take the edge off. The heightened emotions and mood changes you might notice in these weeks are partly down to oestrogen and progesterone surging together, which affects your brain chemistry. There is a clear physical basis to this. Other things in your life play a part too, so if low mood or anxiety lingers, it is worth raising with your midwife or GP rather than putting it all down to hormones. ## How the hormonal picture shifts towards the end of the first trimester In the final weeks of the first trimester, you may find nausea and fatigue start to ease as hCG falls back from its peak and the placenta takes over progesterone. This is a common pattern, though it does not hold for everyone. Some nausea carries on well into the second trimester, and that is also within the range of normal. The first trimester holds some of the fastest hormonal changes of the whole pregnancy. Your body is building the hormonal groundwork that will carry the pregnancy for months ahead, and the physical sensations you feel are a direct result of that work. > **INFO**: This article is for general information and does not replace advice from a healthcare professional. If anything about your symptoms or your pregnancy worries you, speak to your midwife or GP. --- **Q: What are the main hormonal changes in early pregnancy during the first trimester?** Three hormones drive early pregnancy: human chorionic gonadotropin (hCG), progesterone, and oestrogen. Your developing placenta makes hCG soon after implantation, and it is the hormone a pregnancy test picks up. It rises fast in the first weeks, signalling your ovaries to keep making progesterone. Progesterone climbs sharply to hold the uterine lining in place, support the growing placenta, and prevent early contractions. Oestrogen rises steadily alongside them, supporting your baby's organs and the growth of your uterus. By ten to twelve weeks, the placenta takes over progesterone and levels start to settle. **Q: Why does fatigue feel so severe in the first trimester?** The sharp rise in progesterone has a sleep-inducing effect, which is the main reason tiredness can feel so deep even when you are resting enough. Your body is also putting real energy into building the placenta and sustaining the pregnancy. This tiredness is common, and for many people it eases as the first trimester moves on and hormone levels settle. **Q: When does morning sickness usually peak and start to ease?** Nausea and vomiting in early pregnancy is mostly tied to the rise in hCG, and it tends to be at its strongest around weeks nine to ten, when hCG is highest. For many people it starts to ease towards the end of the first trimester as hCG falls back. This is common, though not guaranteed, and some nausea carries on further into pregnancy. If you cannot keep fluids down or symptoms feel severe, speak to your midwife or GP, as this can point to a condition called hyperemesis gravidarum (HG) that benefits from medical support. **Q: Why do mood changes happen in early pregnancy?** The surge of oestrogen and progesterone in the first trimester affects your brain chemistry, which can bring heightened emotions and mood shifts. Other things in your life play a part alongside the hormones. These changes are common, but if low mood or anxiety sticks around, it is worth speaking to your midwife or GP rather than assuming it will pass on its own. ### Clinical Sources - [Physiological Changes in Pregnant Women Due to Hormonal Changes](https://pmc.ncbi.nlm.nih.gov/articles/PMC10993087/) - [Physiological Changes in Pregnant Women Due to Hormonal Changes](https://pmc.ncbi.nlm.nih.gov/articles/PMC10993087/) - [Hormones in pregnancy and labour](https://www.nct.org.uk/information/pregnancy/body-pregnancy/hormones-pregnancy-and-labour) - [Hormones in pregnancy and labour](https://www.nct.org.uk/information/pregnancy/body-pregnancy/hormones-pregnancy-and-labour) - [Week 9 - Your pregnancy and baby guide](https://www.nhs.uk/best-start-in-life/pregnancy/week-by-week-guide-to-pregnancy/1st-trimester/week-9/) - [Hyperemesis Gravidarum](https://www.hey.nhs.uk/patient-leaflet/hyperemesis-gravidarum/) - [Hormones in pregnancy and labour](https://www.nct.org.uk/information/pregnancy/body-pregnancy/hormones-pregnancy-and-labour) - [Physiological Changes in Pregnant Women Due to Hormonal Changes](https://pmc.ncbi.nlm.nih.gov/articles/PMC10993087/) - [HCG Levels In Early Pregnancy](https://www.wwl.nhs.uk/media/.leaflets/602ccb44d38c81.31989740.pdf) - [Hyperemesis Gravidarum](https://www.hey.nhs.uk/patient-leaflet/hyperemesis-gravidarum/) - [Pregnancy mood swings](https://www.clearblue.com/am-i-pregnant/pregnancy-mood-swings) - [Week 9 - Your pregnancy and baby guide](https://www.nhs.uk/best-start-in-life/pregnancy/week-by-week-guide-to-pregnancy/1st-trimester/week-9/) --- Reviewed by Dr Shree Datta, Consultant Obstetrician and Gynaecologist | [Medical Board](https://blog.bloume.com/pregnancy/medical-board.md) | August 2026