The first trimester continues up to the 12th week of pregnancy. During this phase there is a rapid increase in the hormonal levels and a woman’s body undergoes dramatic changes that prepare the body and especially the womb for the growing embryo. The embryo develops from a mass of tissues into a identifiable form with recognizable organs within the initial eight weeks of the first trimester of pregnancy.
Role of hormones during the first trimester of pregnancy:
During the first trimester of pregnancy, there is a surge in the Human Chorionic Gonadotropin (HCG)
hormone, which causes the pregnancy tests to register as positive. This hormone also boosts the levels of oestrogen and progesterone. Oestrogen stimulates development of the foetus, helps the uterus to expand, causes breast enlargement, and boosts blood circulation. Oestrogen, together with oxytocin hormone, prompts uterine contractions, while the increased levels of progesterone hormone maintain the uterus lining and placenta, and relax the uterus.
A woman experiences the following symptoms during the first trimester of pregnancy
- Morning sickness
- Frequent urination
- Changes in the breast – during the first trimester breasts enlarge, and the nipples become darker. Breasts may become heavier and sore. Taking good care and wearing comfortable clothes can provide comfort.
- A slight baby bump
- Constipation and heartburn
Changes during the first trimester of pregnancy- week by week
The embryo is initially a cell that undergoes cell division. Within 3-4 days of pregnancy the cell divides and becomes a mass of around 16 – 32 cells. This cell mass is called morula. During this process the embryo progresses down the fallopian tube and into the uterine cavity. By the end of the first week of the first trimester the morula develops a central cavity with an amniotic sac and a yolk sac. It implants on the uterus lining and is now called a blastocyst.
During the second week of first trimester the embryonic cells begin to differentiate into different cell types. The blastocyst develops an embryonic disc consisting of three primary germ layers – endoderm, ectoderm and mesoderm within its inner cell mass.
- Endoderm – the endoderm cells eventually develop to form linings of the different systems in the body, such as gastrointestinal, respiratory, urogenital tract, liver and various other glands.
- Ectoderm – the ectoderm cells develop into tooth enamel, skin epidermis and sensory organ receptor cells.
- Mesoderm – the mesodermal cells form cartilage and bone, connective tissues of the muscles, some glands, the blood and lymphatic system as well as the dermis.
During this phase of the first trimester, the outer layer of the blastocysts start developing into a placenta. The placenta is responsible for various vital functions like forming a barrier and protecting the baby from harmful foreign substances, providing nourishment to the embryo, and aiding waste removal from the developing embryo. The growing yolk sac provides energy to the rapidly changing embryo until the placenta becomes functional.
During the third week of the first trimester, the 3mm embryo that is attached to the placenta by the umbilical cord and suspended in the fluid filled amniotic sac, develops a neural tube. This neural tube will eventually develop into the spinal cord. An enlarged area at one end of the tubular structure on the embryo, eventually develops into the brain while the other end curls in a tail like structure. The heart muscle fibres also start developing during this stage in a simple pulsating tubular structure.
As the embryo grows it measures around 5mm during the fourth week of the first trimester. During this week of the first trimester the embryo develops rudimentary organs. The heart develops into four identifiable chambers, and starts to beat and pump blood. The basic cartilaginous structure develops and provides structural support. The lungs, gastrointestinal system, kidneys, liver and pancreas develop. The pharyngeal arches start developing and shaping up to form the head and neck. The pits on the sides of the embryo can also be identified; these will eventually develop into eyes. Tiny protrusions that will develop into legs also become visible.
The placenta is established by the fifth week of the first trimester, it becomes functional during this period, and the yolk sac starts shrinking. The hands and limb buds on the embryo lengthen and nostrils can now be identified. The embryo now starts bearing resemblance to a human structure. Special sensory organs like the eyes and the cochlear structure of the inner ear rapidly develop from the neural tissues.
By the sixth week of first trimester the embryo measures almost an inch, as it grows and develops into a human structure. During this phase of the first trimester the hands and limbs start to show a defined structure as they develop fingers. The eyes are now not just two pits but have developed into structures consisting of a lens, retina and eyelids.
7th – 8th week
During the seventh and eighth week of the first trimester the embryo takes a more refined human shape with the head lifted off the chest and a recognisable face, ears, eyes, a protruding nose, neck, well- formed wrists and developing fingerprints. The embryo now measures almost 1.5 inches, and is also able to move. The basic internal organs are now developed and the skeletal cartilages start developing into bones.
10th – 11th – 12thweek
This is the last phase of the first trimester and the embryo can now be refereed to as the foetus. It weighs around 45 grams, and has grown around 3.5 inch long. The foetus’ hands and limbs lengthen and further develop during this phase. The eyes still remain closed but the nervous system and brain are sufficiently developed to sense the pressure on hands and limbs and thus the foetus clenches fists and curls toes in response to stimuli. Since the foetus doesn’t have any underlying fat as yet, the bones that have developed by now are prominent. An ultrasound during the 12th week shows recognisable facial features. During this phase the sexual organs also start developing. However, sex determination at this stage of pregnancy is difficult.
Care during the first trimester of pregnancy
- It becomes essential for a pregnant woman to exercise some basic precautions to keep herself and the developing embryo healthy.
- It is also important to consult a doctor and get the development of the foetus regularly monitored.
- Avoid alcohol, smoking and other recreational drugs.
- Too much caffeine can be dangerous. Unhealthy and unhygienic food, too, should be avoided.
- A nutritious and fibre-rich diet promotes the growth of the foetus and keeps the mother energetic and healthy.
- Your doctor might prescribe some pre-natal vitamins.
- Consult an experienced trainer for pre-natal exercises and work out an exercise regime, accordingly.
- Keep yourself hydrated
- It is important that the woman remains happy during her pregnancy.