The First Trimester of Pregnancy


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This article aims to inform you about what exactly happens during the period of your pregnancy called ‘the first trimester’. This is the first 3 months of a pregnancy, and is typified by many signs that you will be experiencing, as well as particular stages of the development of the newly conceived embryo.

What is the first trimester?

Different authorities define the starting point of a pregnancy (and hence the first trimester) in their own way,  with some using the last day of menstruation as the starting point, while others can claim either the point of fertilisation, embryo implantation, or even ovulation is the true beginning of a pregnancy. Whatever definition your doctor uses, your first trimester will begin from then and continue until about 3 months (12 weeks) from the beginning of your pregnancy.

The word ‘embryo’ is applied during the first 8 weeks of a pregnancy, but from that point onwards the term ‘foetus’ is used. The embryo develops from a ball of cells to a more human structure by the end of the first trimester.

What happens to my body during the first trimester of pregnancy?

Perhaps the best-known sign of pregnancy that is a major nuisance during the first trimester is morning sickness. While this physical sign varies in that some women only experience some nausea, while others experience heavy vomiting, it is a universal sign of pregnancy that comes about as a consequence of elevated hormone levels caused by the pregnancy. Oddly enough, despite the moniker of ‘morning’ sickness, this sign can actually affect women at any time of the day.

Another obvious change is a darkening of the nipples and area around them (called the areolas), also because of a sudden surge in female hormones like oestrogen and progesterone. This will actually continue throughout the pregnancy in preparation for breastfeeding after delivery, and is accompanied by an increase in breast size and sensitivity.

Other changes include a much more pressing need to urinate more often and constipation, two changes that occur as some of your basic bodily functions are altered in preparation to sustain the developing and rapidly growing offspring.

What happens to an embryo during the first trimester?

A fertilised egg makes its way to the embryo and implants during the first trimester, where it triggers the formation of structures that are extremely important to its growth. The most important of these is undoubtedly the placenta, an organ which links the embryo to the mother, providing a means by which nutrition can be provided to the growing offspring. The placenta also takes away waste products, and provides vital oxygen.

During this part of the pregnancy, the embryo’s vital organs begin to develop, and these include the heart and central nervous system (the brain and spinal cord). At this point in the first trimester, about 6 weeks in, the embryo is usually between 4 and 6mm , and a week later features like arms, legs, feet, and face will develop much more quickly. At about the 8th week, the point where rather than an embryo the offspring is referred to as a foetus, movement begins. In the next 3 weeks (up until the end of 12 weeks, the end of the third trimester) a number of other distinct features will present themselves like the toes, fingers, and neck. While the foetus now has all the features of a young child, albeit still in a primitive state, he or she will still be little more than 3 inches long, and can weight as little as 25g.

Important points about the first trimester

The first trimester is actually quite a tricky time during a pregnancy. It is during this twelve-week period that most pregnancies come to an end, and this can be for a whole variety of different reasons. There are many sensitive stages during these first weeks, not the least of which is the implantation of the embryo, which must travel safely from the ovary to the womb, a relatively long distance to travel for a cell no bigger than a full stop.

There are some steps you can take to keep yourself as healthy as possible and thereby give an embryo the best possible chance of survival. These are simple health tips which are covered in more detail in the relevant article, which include ensuring that you have a complete and balanced diet. Supplementing folic acid is crucial to the health of an embryo as it is a vitamin vital to healthy neurological development.

Physical activity is also recommended as fitness helps prepare your body for the stresses of a potential pregnancy. That being said however, overexertion is not recommended as it can be detrimental to foetal health.

Avoiding drugs, alcohol, and smoking is also extremely important. These substances can stress the body and prove detrimental to the health of a pregnancy. There are many campaigns run by the National Health Service that aim to increase awareness about just how dangerous these substances can be for a developing unborn child, and support is available for anyone who wants to quit smoking or drug use in this time.

If you are on prescribed medication when you become pregnant, you should speak to your doctor straightaway as some medications are not recommended during a pregnancy. Your doctor will be able to either recommend an alternative, or suggest the best way of coming off that medication. Stopping completely can be potentially harmful, and therefore you should consult your doctor before you alter your medication habits.


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