First Trimester, Pregnancy

First Trimester Development

Overview | During the first trimester your body undergoes some major changes. You can experience these symptoms even in the very first weeks of pregnancy. Other than a missed period, these are some other changes that you may experience:

  • Extreme tiredness
  • Upset stomach (morning sickness)
  • Mood swings
  • Tender or swollen breasts
  • Cravings or distaste for certain foods
  • Needing to go to the toilet more often
  • Headaches
  • Weight gain or loss
  • Heartburn
  • Constipation

As your body changes, you might need to make changes to your daily routine, such as going to bed earlier or eating frequent, small meals. Fortunately, most of these discomforts will go away as your pregnancy progresses. And some women might not feel any discomfort at all! If you have been pregnant before, you might feel differently this time around. Just as each woman is different, so is each pregnancy.

Week one to three | Your pregnancy is dated from the first day of your last period which means for the first two weeks or so, you’re not actually pregnant. At this stage your body will be preparing itself for ovulation as usual. During the third week, the fertilised egg moves along the fallopian tube towards the womb. The egg starts off as one single cell, which divides again and again. Once it reaches the womb, it has become a mass of more than 100 cells. This is called an embryo. After reaching the womb, it buries itself into the lining of the womb which is called implantation.

Week four | At this stage, the embryo grows and develops within the womb lining. The outer cells reach out to form links with the mother’s blood supply and the inner cells form into two, and then later, into three layers. These will then grow to be different parts of the baby’s body.

The endoderm, or the inner layer, becomes the breathing and digestive systems, including the lungs, stomach, gut and bladder. The middle layer (aka the mesoderm) becomes the heart, blood vessels, muscles, and bones. The outer later, or the ectoderm, becomes the brain and nervous system, the eye lenses, tooth enamel, skin and nails.

Within the early weeks of pregnancy, the embryo is provided nourishment by the a tiny yolk sac it is attached to. A few weeks later, the placenta will be fully formed and will take over providing the nutrients to the embryo. The embryo is surrounded by fluid inside the amniotic sac, and it’s the outer layer of the sac that eventually develops into the placenta. Cells from the placenta grow deep into the wall of the womb, which then gains a rich blood supply enabling all the oxygen and nutrients the baby needs to get to it.

Week five | You will now have noticed you have missed your period, making this week when most women are only just realising they may be pregnant. Yet, at this point the nervous system is already developing in your baby and the foundations for their major organs are in place. The embryo will measure around 2mm long at this point.

As the ectoderm develops, a groove forms and the layer of cells folds to form a hollow tube. This is known as the neural tube and will become the baby’s brain and spinal cord. A defect in the ‘tail end’ of the neural tube can lead to spina bifida and defects in the ‘head end’ lead to anencephaly.

The heart is also now forming as a simple tube-like structure and your baby already as some of its own blood vessels and blood circulating. A string of these blood vessels connects the baby and the mother, which will become the umbilical cord.

Week six | There is now a significant bulge where the heart is and a bump at the ‘head end’ of the neural tube, which will become the brain and head. The embryo is curved and has a tail – often described as looking like a small tadpole at this point. The arms and legs which are now developing become visible as small swellings. Little dimples form on the side of the head which will become the ears, and there are thickenings where the eyes will be. One notable point at this stage is that the embryo is covered with a thin layer of see-through skin.

Week seven | The embryo now measures about 10mm long from head to bottom and the brain is growing rapidly which makes the head grow faster than rest of the body. It now has a large forehead, and the eyes and ears continue to develop. The outer ear on the side of the head won’t appear for a couple more weeks, but the inner ear begins to develop now.

The limb buds start to form cartilage which develops into the bones of the legs and arms and the ends of the arm buds will flatten out to form the hands. The nerve cells continues to multiply and develop as the nervous system starts to take shape.

Week eight | Once you reach eight weeks, the baby is now called a foetus. The legs are lengthening and forming cartilage too. The parts of the leg aren’t distinct yet, but in time the knees, ankles, thighs, and toes will develop.

The foetus remains still in the amniotic sac and the placenta is continuing to develop. The structures called chorionic villi are forming which will attach the placenta to the wall of the womb. It is still receiving its nourishment from the yolk sac.

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