The First Trimester describes the period of time from the first day of your last period to the end of the 12th week. It can be confusing to be told that when you first find out you are pregnant you may be at least 4-5 weeks pregnant. This will be approximately 2-3 weeks from the time you may have had intercourse and time of conception.
Fertilisation is the process of the sperm and egg joining together.
The fertilised egg (oocyte) divides into two equal size cells and after dividing three more times there are 16 cells.
The Blastocyst is formed within a week and comprises of Trophoblasts (outer cells) that make contact with the lining of the uterus (endometrium) assist in implantation and form the placenta. The inner cells (embryoblast) form part of the Blastocyst and form the embryo.
During the second week, further complex development takes place and specialised cells are made.
The embryo implants (Implantation) into the lining of the uterus (endometrium) following the previous process of cell division. This occurs around 7 days after fertilisation. Human chorionic gonadotophin (HCG) can be detected in the blood but may not be detected in the urine yet. HCG maintains the corpus lute in the ovary (from where the egg was released) and progesterone is produced the maintain the pregnancy.
Three layers of cells form different tissues of the foetus (Gastrulation). This occurs around day 14 from fertilisation. The endoderm layer of cells are responsible for digestive and respiratory tracts, urethra, bladder and reproductive system and the liver and pancreas.
Mesoderm cells enable development of the circulatory system, musculoskeletal system and muscles of the stomach and intestine.
The Ectoderm layer of cells are responsible for the development of the skin, nervous system the cornea and lens of the eye.
Then comes the development of the embryo’s nervous system.
Between 6-7 weeks organogenesis takes place and organ development takes place. The heart, limbs, eyes, upper lip and intestines are developing.
This is the best time to come to Somerset Early Scans for an early scan, 7 weeks from the first day of your last period. The heartbeat is more reliably seen at 7 weeks due to many women not having exactly 28-day cycles.
Between 7-8 weeks whilst organogenesishas almost completed, brain development, external genitalia and completion of the palate is still ongoing.
By 12 weeks the baby is approximately 6cm in length. The first trimester is coming to a close. Internal organs and muscles are developed, and the skeletons soft tissue will be hardening into bone.
You will now enter the 2nd Trimester of your pregnancy.