Final answer:
The first trimester is the beginning phase of a pregnancy that lasts for about 12 weeks and involves the onset of various signs and symptoms, alongside the critical development of the fetus's basic structures. It is a vital period where the placenta forms, enabling nutrient transfer and passive immunity for the fetus.
Step-by-step explanation:
The first trimester of pregnancy, or the first 90 days, is referred to as the initial phase of gestation starting from the time of fertilization and lasting for the next 12 weeks. During this period, the pregnant person may not even be aware of their pregnancy, yet they could start experiencing various signs and symptoms such as a missed menstrual period, tender nipples, increased appetite, frequent urination, and what is often termed "morning sickness", which includes nausea and vomiting at any time of day, despite the name. The first trimester is crucial as it lays down the foundation for the developing fetus, forming basic structures like limb buds, heart, eyes, and the liver.
On average, the duration of pregnancy is 40 weeks, divided into three trimesters. Each trimester has its own set of developmental milestones and physiological changes in both the fetus and the pregnant person. The first trimester is essential for the establishment of vital organs and structures, which pave the way for further development in subsequent trimesters.
In addition to the physical developments, the gestational process involves the formation of the placenta during the first trimester. This organ plays a critical role in transferring nutrients and waste between the fetus and the mother, and also enables the passage of some immunoglobulins, providing the fetus with passive immunity.