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Alison Ross
Registered Midwife, DipHe, BSc (Hons) Was a midwife at Kingston Hospital and Specialist Midwife in Perinatal Mental Health.
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What are the three trimesters of pregnancy?

The three different "trimesters" pregnancies are divided into, generally follow quite specific patterns - both for the way the Mum feels, and for the development of the baby.
In Short
The first trimester is the first third of the pregnancy or weeks 1-12.

The second trimester is the second third or weeks 13-27.

The third trimester is the last third or weeks 28 to 40 (the birth), although a baby is considered full-term from 37 weeks.

What are the three trimesters of pregnancy?

Your pregnancy officially starts from the first day of your last menstrual period. Obviously, it might really start quite a bit later than that, you might even know exactly the date your baby was conceived. Your dating scan can also help determine the real date as well if you don’t already know it.

The whole pregnancy lasts around 40 weeks or 9 months.

The first trimester is the first third of the pregnancy or weeks 1-12.

The second trimester is the second third or weeks 13-27.

The third trimester is the last third or weeks 28 to 40 (the birth), although a baby is considered full-term from 37 weeks.

What’s happening with your baby in each trimester?

During the first trimester, babies grow at a tremendous rate. They start as a few cells, at which point the Mum might not even be aware she’s pregnant. Babies will then attach to the wall of the uterus in a process called implantation. By the end of the first trimester, all going well, they will have grown to about 6-8 cm in length and weight about 1 ounce.

During this stage, the amniotic sac, umbilical cord, and placenta grow.

All the major organs in babies will start to develop, as well as their neural tubes, and their hearts will start to beat on around day 25.

Little buds will form, which will develop into limbs. Even fingers and toes, ankles and ears will start to develop.

During the second trimester, babies will probably start to move around and kick more. By month 5, they can usually turn from side to side and even go upside down. They will also start to have periods of being asleep, which Mums will probably learn to recognize as well. By the end of this trimester, babies are usually around 20-30cms long and weighing about 1 lb.

During the third trimester, babies can open and close their eyes, and respond to sound and light. There is less and less room in the womb for them to move, and they tend to move into the birth position, head down, and stay there, tightly packed in .

How will you feel in each trimester?

How mums feel in each trimester varies enormously from woman to woman. Some Mums will sail through all three stages with no nausea and little tiredness. Others will feel morning sickness from the very beginning, right through to the birth. Others will have it only in the first trimester.

You may find you need to pee a lot more often, or you may go off certain foods. Or not, some Mums have no change in appetite or taste.

Your breasts usually swell and become a little tender. Or a lot.

You may get out of breath more easily, especially as the pregnancy progresses since your baby will start to push up into your lung space.

Usually, the second trimester is the easiest. This is when women feel like they are blooming, their energy levels return to normal, they have a small comfortable bump and often don’t feel as sick or tired as before or afterwards.

The first trimester can be exhausting as your body changes physiologically to grow and develop your baby. It is also in these first 12 weeks that women report that their morning sickness and food preference changes peak.

The third trimester can become increasingly tiring, carrying around a much bigger baby.

By the third trimester, you might start to feel quite uncomfortable, with heartburn, haemorrhoids, swollen ankles, warm body temperature, and tiredness. Speak to your midwife if you have excessive ankle swelling that is not normal for you. Your midwife can check your blood pressure and rule out pre-eclampsia.

Your breasts may put on another final growth spurt and as the breast tissue develops your breasts may leak colostrum (this happens earlier in some pregnant women whilst others will not see any colostrum until after their baby is born). This colostrum production is getting ready for feeding your baby.

Other external changes include the appearance of stretch marks due to the rapid growth of your abdomen that occurs in the third trimester as your baby starts to put on weight and grow in size.

Despite all these changes and discomfort, the third trimester is a period of great excitement and planning, as the birth of your baby is imminent. This is when your nesting might really kick in and you will feel a burst of energy as you plan and prepare for your baby’s arrival.

In the final trimester, your midwife will be seeing you regularly to ensure the on-going health of you and your baby and giving you the opportunity to ask questions that are concerning you.

It’s important to monitor your baby’s movements at this time and get used to their rhythms so you can see your midwife if your baby’s movements change. An easy thing to do if you are worried is lie down quietly after eating and count 10 movements by your baby.

Reduced movement can be your early warning that your baby needs to be born a bit earlier than planned.

These are general patterns but it can be a little bit different for everyone. You are the expert in yourself and your pregnancy so try to listen to your body at each stage and be kind to yourself.

Watch our video where mum, Melissa, describes how she felt in each trimester.

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This article is for information only and should not be used for the diagnosis or treatment of medical conditions. Essential Parent has used all reasonable care in compiling the information from leading experts and institutions but makes no warranty as to its accuracy. Consult a doctor or other health care professional for diagnosis and treatment of medical conditions. For details click here.