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First Trimester

Updated on September 4, 2009

The first trimester begins from two weeks after your last period. The fact that you probably won’t realise – and may go a few weeks before you do – is irrelevant. The length of your first trimester lasts up to and including your twelfth week. It’s also the stage of your pregnancy whereby your baby will do the most developing.

The First Trimester

From the start of the first trimester, your baby is readying himself for his future birth. This also influences you, your body and your general health and well-being. Due to all the changes that your baby undergoes your body will have to compensate, create the perfect balance, in order for your baby to be healthy, happy and fully developed in time for his eventual birth.

From the moment of conception onwards, he is totally reliant on you, so that all his developmental needs are met – quite a tall order! However, as women, we’ve been undergoing the pregnancy process for thousands of years and at least in today's world, there’s lots of health and advice related to pregnancies - not to mention health professionals at hand.

First Trimester - Your Baby

In the first trimester, after an egg has been fertilized, it becomes what’s known as a zygote – this is due to the name of the rapid multiplication of the original two cells: the egg and the sperm = the zygotic stage. It will continue travelling down the fallopian tube, all whilst the cells continue to multiply at a rapid rate.

As it enters your womb to implant into the lining, it becomes a blastocyst. At this stage, all the outer cells will form the placenta and amniotic sac and the inner ones will become the embryo. Over the next few weeks, the blastocyst will continue to grow and develop, so that by the fifth week it has become an embryo – and the heart, spinal cord brain and other major organs begin to form.

First Trimester - Conception To 12 Weeks

Click thumbnail to view full-size
Conception to implantation in the womb.4 weeks gestation8 weeks gestation12 weeks gestation
Conception to implantation in the womb.
Conception to implantation in the womb.
4 weeks gestation
4 weeks gestation
8 weeks gestation
8 weeks gestation
12 weeks gestation
12 weeks gestation

As the weeks progress, many major changes will occur:

· the basis for your babies’ circulatory system

· central nervous system

· basic facial features

· primary limb formation

· eyes and nostrils begin to form

· genitals begin to develop

As the first trimester draws to a close, your baby will have gone from the conception stage to a recognisable fetus. He will be around 7/8 grams in weight and perhaps two inches in length. Although there’s still a long way to go – the foundations have now been laid. Six months on from this stage of his development, he’ll be a perfectly formed, bouncing baby. And very probably – a crying one too!

First Trimester - You


First Trimester And You

Morning sickness - can be an early sign, as well as the bane, of pregnancy.
Morning sickness - can be an early sign, as well as the bane, of pregnancy.

As previously stated, your first trimester usually begins before you’re even aware of it. However, the great part is – you don’t need to be. Your body will respond as nature intended and things will tick along, irrespective of whether you’re conscious of it or not. Right from the start, you’re body will be awash with hormones and it’s usually these that give some women their first inkling that they may have conceived.

Hormone levels affect our moods and pregnancy brings a positive flood - therefore you may find that you’re more emotional than usual. Other little symptoms that may start to become apparent in the early days of your first trimester are:

  • nausea or ‘morning sickness’
  • cravings
  • breast tenderness
  • fatigue

And the most obvious sign of being pregnant – a missed period. The next step of course is to have your pregnancy correctly diagnosed by way of a simple pregnancy test. However, the changes in your body will carry on regardless. And hopefully whatever your body is doing in the absence of your possible lack of awareness - won't include morning sickness!

The first trimester brings breast tenderness for many women, as well as the dreaded morning sickness. Generally, the nausea will pass as your pregnancy progresses but for many women, the first few months can be a little difficult. You may also experience an increased need to urinate, even though your uterus is still quite small. This is due to the hormone progesterone - and little to do with the size of your womb. As the weeks progress, so does the volume of blood circulating around your body.

Many women can’t regulate their internal ‘heating system’ quite as easily so can feel or look a little flushed - and this is due to the extra blood volume.  At some point during the first trimester you’ll attend your first of many antenatal appointments. You may find that your waist and mid-section expand a little during the first few months, though it tends to be more of a thickening than anything resembling a baby bump. Your breast will have begun to increase in size and it’s worth investing in maternity underwear as soon as you feel you need them.

By the time you’ve completed your first trimester, it’s amazing just how much your baby and you have already accomplished. All being well, with the correct care and attention to your health and well being, you and your baby will hopefully experience a trouble free journey together.

First Trimester - Onto The Second


If you’re reading this and you’re newly pregnant, congratulations. Alternatively, if you’re nearing the end of your first trimester and are about to enter your second stage of pregnancy – you and your baby are about to undergo just as many changes as you’ve both experienced in the first few months so – take things easy!


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