Trimester One – Weeks One through Six

The first trimester of pregnancy is an exciting time full of changes and adjustments.  During the first several weeks of pregnancy, the fetus forms its most basic organ systems, and a woman's body undergoes complex changes that can cause an array of pregnancy symptoms.  Understanding fetal development and first-trimester symptoms will help women get through the initial weeks of pregnancy.

Early Pregnancy Fetal Development

Week One

When the sperm penetrates the egg, it forms a cell that divides and travels toward the uterus.  The cell group implants onto the uterine wall around day six, at which point it is referred to as an embryo.  Implantation signals the beginning of pregnancy.

Week Two

During week two, the cells of the implanted embryo multiply and differentiate.

Week Three

The fetus's brain, spinal cord, heart, and digestive tract begin to form.

Week Four

Rudimentary blood vessels begin to form, and the heart begins to beat.

Week Five

The circulatory system further develops and becomes functional.  The digestive and respiratory systems, arm and leg buds, and eye and ear structures begin to form.

Week Six

The fetus is about an eighth-of-an-inch long.  Organs and facial features become more well-defined. The neural tube closes, which will later differentiate the brain, spinal cord, and nerves.

First Trimester Pregnancy Symptoms

Once the embryo is implanted on the uterine wall, the body begins producing hormones that can trigger symptoms including:

• Tender, swollen breasts
• Fatigue
• Slight cramping or spotting
• Nausea and/or vomiting
• Food cravings or aversions
• Headaches
• Constipation
• Mood swings
• Faintness
• Increased body temperature

Continued Reading:

Trimester One - The Second Half