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.1
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.2
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.3
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.45
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.10
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.7
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.812
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.