The first trimester starts on the first day of your last period until the end of week 12. You might already be 5 or 6 weeks pregnant when you find out.
My first trimester of pregnancy: joy, worry, extreme tiredness and depression — Noémie Memories
The First Trimester of Pregnancy
First Trimester: Symptoms and Baby Development
First Trimester: What to Expect?
First trimester FAQ: Everything you need to know about these early weeks - Today's Parent
First Trimester of Pregnancy (Week 1–Week 12) - StoryMD
Stages of Pregnancy by Trimester
First Three Months Of Pregnancy: 14 Crucial Precautions To Take
Pregnancy Chart: What to Expect Each Trimester
Pregnancy and the first trimester.
What to Do and Avioid in the First Trimester of Pregnancy?