The first trimester of pregnancy is often considered the second most exciting time during pregnancy. Birth is, of course, the most exciting time. During the first trimester of pregnancy, women find out they are pregnant, attend the first prenatal appointment and notice a long list of changes in their body. This usually happens during the first trimester doctor's visit. Understanding what happens in the body during the first trimester of pregnancy is important — because some of these changes may not be quite what you expected!

First trimester week-by-week

1. Week 1
2. Week 2
3. Week 3
4. Week 3
5. Week 5
6. Week 6
7. Week 7
8. Week 8
9. Week 9
10. Week 10
11. Week 11
12. Week 12
13. Week 13

What happens at the beginning of the first trimester of pregnancy?

Even before the pregnancy test is positive, there are changes happening during the first trimester of pregnancy. During ovulation, the egg is released and the corpus luteum signals the release of progesterone. Progesterone causes the lining of the uterus to thicken and engorge with blood. When fertilization occurs, the fertilized egg implants in the thicker lining and starts to grow immediately. Human chorionic gonadotropin (hCG) is released keeping the corpus luteum alive for the duration of the pregnancy. hCG is the hormone that turns a pregnancy test from negative to positive.

How do these first trimester changes affect you?

While women do not physically feel the egg and sperm meet, they may feel the egg implanting in the uterus. Some women report feeling a small cramp and a bit of spotting. This spotting is typically associated with the upcoming menstrual cycle when it is actually implantation spotting.

So, the hormones estrogen, progesterone and hCG are flooding the body at this point. From the first moments after conception, the body kicks into a gestational mode. These hormones are to blame for many of the pregnancy-induced symptoms women feel in the first trimester of pregnancy including nausea, vomiting, and fatigue. Some women actually feel like they have a touch of the flu for most of the first trimester of pregnancy. Hormones are not selective. They cannot increase in the bloodstream and selectively attend to only the needs of the fertilized egg (zygote and later fetus). When progesterone levels rise, you will feel symptoms more.

Learn how a healthy pregnancy diet can reduce birth defects!

The human body is highly adaptive, however, after the first trimester of pregnancy, things should go back to normal for a while. The morning sickness and tender breasts may be fading a bit or it could be just that you're getting used to feelings and symptoms due to hormone surges. For many women, the first trimester of pregnancy does not last very long at all because many pregnancies are not detected until after the 6th week of gestation. That leaves only 6 weeks to conquer the first trimester of pregnancy before moving into the period of growth and more changes.

First trimester tools, calculators, and quizzes

• Am I Pregnant? Online Pregnancy Quiz
• Early Pregnancy Signs and Symptoms Test
• Fertility and Ovulation Calculator and Calendar
• Is My Menstrual Period Late? Calculator
• Is This Implantation Bleeding?
• Pregnancy Due Date Calculator
• Pregnancy Calendar and Calculator Week by Week

What you need to know about your first trimester

• First Trimester Checklist
• How to Survive the First Trimester of Pregnancy in 12 Steps
• What Happens in the First Trimester of Pregnancy With My Body?
• What Happens in the First Trimester of Pregnancy With My Baby?
• What Happens in the First Trimester of Pregnancy with Doctor's Appointments and Tests?
• First Trimester of Pregnancy and Your Relationship
• First Trimester Screen Test - Nuchal Translucency
• First Trimester Ultrasound Scanning
• First Trimester from the Male Perspective
• First Trimester Miscarriage

Other trimesters

1. Pre-Trimester (preconception)
2. First Trimester (up to 12-13 weeks)
3. Second Trimester (up to 26-27 weeks)
4. Third Trimester (up to delivery)
5. Fourth Trimester (postbirth or postpartum)
6. Fifth Pregnancy Trimester (going back to work)