How your body and hormones change in the first trimester of pregnancy

All you need to know about your baby, your body, and your hormones during weeks one to 12 of pregnancy.

5 min read

How your body and hormones change in the first trimester of pregnancy

All you need to know about your baby, your body, and your hormones during weeks one to 12 of pregnancy.

5 min read

What can I expect in the first 12 weeks of pregnancy?

The first few weeks of pregnancy are a steep learning curve. Find out how your body will change during pregnancy and learn what to expect during your first trimester.

What is the first trimester and how long is it?

For ease, pregnancy is divided into three periods and the first 12 weeks is called the first trimester, lasting roughly three months. After the first 12 weeks, from 13 to 26, is the second trimester. The entire length of pregnancy is described as 40 weeks, dating from the first day of your last period, up to the expected date of your baby’s birth.

What happens to my body and my baby in the first trimester?

As your body is changing rapidly to grow a perfect little baby, it’s inevitable that there will be several changes to both your body and to the baby growing inside you. Here’s what to expect:

What happens to your body:

  • Missed period. The first sign of pregnancy may be a missed period, though if your periods have been irregular, you may not realise at first that you are pregnant.

  • Changes to your breasts. You may find that your breasts feel more tender or sensitive than usual, they may feel swollen or sore.

  • Nausea. You may get a feeling of nausea or be sick. This is known as morning sickness, though it can happen at any time of day.

  • Needing to wee. Your uterus is growing and pressing on your bladder, making you want to wee all the time. You also have extra fluid in your body which adds to this need.

  • Tiredness. Hormones are soaring and they make you very tired.

  • Cravings. You may find that you long for certain foods. You might also have an aversion to some foods and smells.

  • Indigestion. The pregnancy hormones relax your digestive muscles, meaning that heartburn is more likely.

  • Constipation. Progesterone makes your digestion more sluggish, leading to constipation.

  • Emotions. The hormones plus the tiredness, plus mixed emotions about the pregnancy, can leave you very tired and with your feelings being high one minute and low the next.

What happens to your baby:

  • Name. Your baby is an embryo for the first seven weeks, then is called a foetus from this point on.

  • Features. Your baby’s nose, eyes, ears, and spine develop during this time and the tiny feature that will become the heart starts to form and then to beat.

  • Organs. Your baby has all its major organs by eight weeks.

  • Other features. Teeth begin to appear by around 12 weeks, as do finger and toenails, kidneys, and external genitalia.

  • Movements. Your baby can start to move by 12 weeks, though you may not be able to feel it just yet.

First trimester pregnancy milestones

Here are a few of the milestones you’ll be expecting to pass during your first trimester:

The key stages during the first trimester for your baby

  • Four weeks: A sperm meets an egg, and the egg is fertilised. The fertilised egg, called a blastocyst, passes through the fallopian tube to the uterus

  • Five weeks: your baby’s heart is formed and begins to beat

  • Six weeks: some of the features are formed and arm and leg stumps start to form

  • 10 weeks: This is when your baby changes from an embryo to a foetus. Organs have been formed and nails start to grow.

  • 12 weeks: by this time, you can hear your baby’s heartbeat on an ultrasound scan.

The key stages during the first trimester for you

  • Missed period

  • Positive pregnancy test

  • Sharing the exciting news with your partner and perhaps close friends and family

  • First antenatal visit to your doctor to get ‘booked in.’

  • First feelings of morning sickness

  • First craving – or avoiding certain foods!

  • First ultrasound scan

  • Hearing baby’s heartbeat

  • Sharing the news with a wider circle of friends and family

What should I avoid during the first trimester?

Because your baby is growing and developing so rapidly, they can be affected by hazards you might be exposed to. Avoid smoking altogether and stop taking any recreational drugs. Discuss any regular prescription drugs you take with your doctor to ensure they are suitable for pregnancy. Avoid alcohol and X-Rays. Try not to get infections, so avoid food that’s past its best before date or food that is not safe for pregnant women.

What can I do to help during the first trimester?

It’s important to keep as well as you can and to help yourself when your body is going through so much. Take regular rest, try to avoid stress, and eat well. Exercise that’s safe for pregnancy is great and can help you both physically and emotionally. Make sure you are taking a good pregnancy vitamin that contains folic acid.

Can I upset the baby in the first trimester?

There are certain foods to avoid during pregnancy because they may lead to food poisoning, which can be very dangerous to your unborn baby and might cause a miscarriage. There are other foods that are too high in certain vitamins or minerals. You should also avoid any sports or activities that might lead to a fall, which could damage your baby. There are, however, plenty of exercises you can still enjoy while pregnant.

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