firsttrimester

The first trimester is the initial phase of pregnancy, spanning roughly the first 12 weeks of gestational age. Gestational age is counted from the first day of the last menstrual period, though fertilization usually occurs about two weeks after that date. This period involves rapid development as the foundations for the fetus’s organs and systems are established.

Key developmental milestones occur between weeks 3 and 12. Organ formation (organogenesis) takes place, the neural tube closes by weeks 4 to 6, and the heart begins beating around weeks 5 to 6. By the end of week 12, most major organs have started to develop and the embryo is typically referred to as a fetus. The fetus remains small, but growth proceeds rapidly.

Common symptoms during the first trimester include nausea and vomiting, fatigue, breast tenderness, frequent urination, and mood changes. Symptom intensity varies widely among individuals, and some people have few noticeable signs.

Prenatal care usually begins in the first trimester. The initial visit often occurs around 8 weeks and includes medical history, blood and urine tests, blood type and Rh status, infection screening, and a dating ultrasound. Optional screenings in this period may include an early ultrasound for dating and, around weeks 11 to 14, a nuchal translucency test to assess certain chromosomal conditions.

Lifestyle and safety guidance focuses on nutrition, safety, and avoiding toxins. Daily folic acid supplementation is recommended (typically 400–800 micrograms). A balanced diet, caffeine moderation (generally under 200 mg/day), avoidance of alcohol and tobacco, and careful use of medications are advised. Certain foods to avoid include unpasteurized products, undercooked meats, and high-mercury fish.

Risks during the first trimester include miscarriage, which is more common early on. Seek prompt medical attention for heavy bleeding, severe abdominal pain, dizziness, fainting, or fluid leakage.