What is implantation that occurs when u are pregnant and what is first trimester?

Implantation: Implantation occurs about 6days post fertilization or cycle day 20 or so. Implantation is the embryo starting to invade the endometrium becoming fixed and immovable. The placental cells and vessels join those of the endometrium to feed the fetus. Bleeding this process continues to the time of placental maturity, about 12 - 15wks of pregnancy. First trimester is through 12weeks.

Early pregnancy: After ovulation, if the egg is fertilized, implantation of the embryo occurs day 6 1/2 to 7 days later. This is when the embryo attaches to the uterine wall and starts to connect with the maternal blood supply. The early placenta (that will eventually nourish the baby) starts development at 7 weeks and is complete by the end of the 12th week; which is also the end of the first trimester.