There are three trimesters in pregnancy and each has different developmental characteristics. The estimated due date (EDD) is set by calculating 40 weeks from the first day of the last monthly period (LMP); which means that when fertilisation occurs the woman is already two weeks pregnant.
For a detailed view of foetal development week by week – with pictures! Please click here.
In the first trimester, when an ovum (egg) is released it travels down the fallopian tube towards the uterus; the corona radiata (outer layer of the ovum) contains follicular cells that are difficult to penetrate; spermatozoa (sperm) must then secrete a digestive enzyme in order to weaken the corona radiata. Once the sperm have penetrated the egg the chromosomes combine to form a zygote (a one-celled body which contains 46 chromosomes); hCG is then released to increase progesterone levels and stop your period. The zygotes’ cells divide rapidly until there is a cluster of 16 cells (a morula); fluid collects within the morula creating a separate outer layer which encapsulates the inner cluster of cells, the inner layer will eventually become the embryo and the outer layer will form the placenta; there are now 58 cells in the structure and it is now called a blastocyst.
The blastocyst then burrows into the uterine wall (implantation). During the embryonic period (weeks five to ten) the major structures begin development; there are three layers to the embryo; the outer layer (ectoderm) forms the outer layer of skin, nervous systems, eyes, inner ears, and connective tissues; the middle layer (mesoderm) forms the heart and circulatory system, along with the bones, muscles, kidneys and the reproductive system; and the inner layer (endoderm) becomes a tube lined with mucous membranes ready for the development of the lungs, intestines and bladder.
While the placenta is forming the embryo is nourished by the yolk sac. The brain forms and the heart is starting to pump blood through the main blood vessels; the tissue which will become the spine is growing and has developed somites; the eyes are beginning to form, and arm and leg buds are developing. The neural tube then closes and the ears and nostrils begin to develop; the lungs are also forming. By the seventh week the arm buds look like paddles which will develop into fingers. The spine eventually begins to straighten and as the arms continue to grow they can bend at the elbows; toes then start to form and all of the essential organs have begun to grow. The eyelids then fuse shut, and the intestines rotate. At the end of week ten the embryo is termed a foetus. Red blood cells begin to form in the liver, tooth buds appear and the external genitalia starts to develop into either a penis or clitoris. By the end of this trimester the embryo has the appearance of a miniature human.
During the second trimester the intestines (which have been growing in the umbilical cord) return to the abdomen and bone tissue is developing. The ovarian follicles start to form in females and the prostate appears in males. Red blood cells are developing in the spleen and bones have also begun developing, with movements becoming better coordinated; the ears move near to their final position and the foetus can hear sounds outside of the womb; the mouth now makes sucking motions. Fat stores form under the skin, and vernix (a greasy coating) covers the foetus offering protection from abrasions and chapping due to the amniotic fluid. The next stage of development is the swallowing reflex; the foetus swallow’s amniotic fluid and then urinates before swallowing again, this helps to mature the lungs; lanugo (soft, fine hair) covers the foetus helping to keep the vernix in place; and meconium is now made in the intestinal tract. The foetus begins to show signs of rapid eye movements, fingerprints are starting to form and taste buds are developing. In females the uterus and ovaries which contain a lifetime supply of immature eggs are in place and in males the testes have started to descend from the abdomen. Bone marrow is now making blood cells and the startle reflex is developing; the foetus may respond to sounds with movement. Surfactant is now being produced in the lungs, allowing the air sacs to inflate and deflate.
In the third trimester the foetus rapidly gains weight; the bones are fully developed although still soft, and red blood cells have formed in the bone marrow. The eyes open and the pupils can now detect light; the brain is growing rapidly and during the thirty-first week the central nervous system can control body temperature. The lungs begin to practice breathing, lanugo starts to disappear and the foetus begins to absorb minerals (iron and calcium) from the intestinal tract. At thirty-seven weeks the foetus is classed as early term, all organs are able to function and the head may begin its descent into the pelvis. The foetus is considered full term at 40 weeks.