The days after birth can feel like the ground has shifted under you. The tearfulness that arrives without warning, the tiredness that sits in your body differently from ordinary tiredness, the irritability that catches you off guard. These come from a rapid change in your postpartum hormones in the fourth trimester, and understanding what is happening can make it all feel less bewildering.
The hormonal shift after birth
During pregnancy, the placenta makes most of your oestrogen and progesterone, the hormones that keep the pregnancy going and support your growing baby. Within hours of the placenta being delivered, those levels fall sharply. Progesterone usually stays low until your periods come back. This sudden drop is the main trigger for what is often called the baby blues.2
In the first few days after birth, you may notice tearfulness, irritability, and anxiety that feel out of step with what is going on around you. This is the baby blues, and it is very common, affecting up to 80% of mothers. These feelings are a normal response to the hormonal change your body has just been through, and they usually ease within two weeks. If anything worries you along the way, your midwife or health visitor can talk it through with you.3
What prolactin does in the fourth trimester
As oestrogen and progesterone fall, another hormone, prolactin, rises. Prolactin is what stimulates and keeps up your milk supply. If you are breastfeeding, each time your baby feeds, your pituitary gland (a small gland at the base of your brain) releases more prolactin, which signals your body to make more milk. That is the supply-and-demand pattern behind milk production.4
While you are breastfeeding, high prolactin often holds off ovulation, so your periods may not return for a while. This is worth knowing: it is not a reliable form of contraception. You can ovulate before your first period arrives, which means pregnancy is possible even before your cycle resumes. Your GP, midwife, or health visitor can talk you through postnatal contraception and help you find an approach that suits you.1
How these changes affect mood and energy day to day
The drop in oestrogen and progesterone lines up with mood swings, fatigue, irritability, anxiety, and sadness. Broken sleep and the demands of caring for a newborn can pile on top of that. So the early weeks often feel harder than you expected. This is your physiology adjusting.5
For most people, these feelings ease as the first two weeks pass. If low mood, hopelessness, anxiety, or trouble bonding with your baby carries on past that point, it is worth speaking to your GP or health visitor. Postnatal depression (PND) affects more than 1 in 10 women and can be treated. A healthcare professional can assess what you are going through, so this is not something to weigh up or sit with on your own.6
A less visible hormonal change: the thyroid
There is another hormonal shift that gets less attention: postpartum thyroiditis. This is inflammation of your thyroid gland, the gland in your neck that helps regulate your energy and metabolism. It can show up in the first year after birth and affects around 5 to 10% of women. It often goes unnoticed because its symptoms overlap with the baby blues and PND.7
Postpartum thyroiditis often starts with anxiety, irritability, and a racing or pounding heartbeat, then moves into a second phase with fatigue, low mood, feeling cold, and trouble concentrating. These are examples of how it can feel, not a diagnosis. If any of these stick around or start affecting your daily life, a GP can check your thyroid with a straightforward blood test.8
Practical things worth knowing
The fourth trimester is a time of big physical and emotional adjustment. Your body has just been through a major hormonal change, and the early weeks are when your postnatal care, from your midwife, health visitor, and GP, matters most. If something feels persistently off, or you are unsure whether what you are feeling sits within the normal range, call your midwife or health visitor rather than waiting for a scheduled appointment.
If you are breastfeeding, your calorie and nutritional needs are noticeably higher than usual. It is worth keeping that in mind with how you are eating, and your health visitor or GP can give you more specific guidance.