Stages of Pregnancy
Stages of Pregnancy
Pregnancy is divided into three trimesters, each with distinct phases of fetal development and maternal change. Understanding what happens at each stage helps you make informed decisions about nutrition, supplementation, and self-care.
First Trimester (Weeks 1–12)
Fetal development
The first trimester is the most critical period for fetal development. By week 6, the heart is beating. By week 8, all major organs have begun to form. Neural tube closure — which determines brain and spinal cord development — occurs by week 6, making early folate intake essential. By week 12, the fetus is fully formed and approximately 6cm long.
Maternal changes
hCG (the pregnancy hormone) rises rapidly, causing nausea, fatigue, and breast tenderness. Blood volume begins to increase. The uterus grows but is not yet visible externally for most women.
Key nutritional priorities
- Methylfolate: neural tube closure
- Vitamin D: immune support and early bone development
- B6: may reduce nausea
- Iron: begin building stores early
- DHA: early brain development
- Choline
Second Trimester (Weeks 13–26)
Fetal development
The second trimester is often called the ‘golden period’ of pregnancy. The fetus grows rapidly — from around 7cm at week 13 to approximately 35cm by week 26. Movements become detectable (quickening) from around weeks 16–20. The fetus develops hearing, and the lungs begin to mature.
Maternal changes
Nausea typically eases. Energy levels often improve. The bump becomes visible. The cardiovascular system continues to adapt — blood volume increases by up to 50% by the end of pregnancy. Heartburn, back pain, and round ligament pain become more common.
Key nutritional priorities
- Iron: blood volume expansion accelerates
- Calcium: fetal bone mineralisation begins
- Magnesium: muscle function and sleep
- DHA: brain and eye development continues
- Vitamin C: collagen synthesis and iron absorption
Third Trimester (Weeks 27–40)
Fetal development
The third trimester is dominated by growth and maturation. The fetus gains approximately 200g per week. DHA accumulates rapidly in the brain. The lungs mature in preparation for breathing. The fetus moves into a head-down position (usually by week 36) in preparation for birth.
Maternal changes
Physical discomfort increases as the uterus expands. Shortness of breath, pelvic girdle pain, swelling, and sleep difficulties are common. Braxton Hicks contractions become more frequent. The body begins preparing for labour — the cervix softens and the baby engages in the pelvis.
Key nutritional priorities
- DHA: peak accumulation in fetal brain
- Iron: highest requirements in third trimester
- Vitamin K2: fetal bone mineralisation
- Magnesium: leg cramps, sleep, and preterm labour prevention
- Vitamin D: immune function and calcium absorption
The Fourth Trimester (Weeks 1–12 postpartum)
The fourth trimester refers to the 12 weeks after birth — a period of profound physical and emotional adjustment for the mother. Nutritional needs shift significantly, particularly if breastfeeding. See our Postnatal Health pillar for full guidance.
Frequently asked questions
How is pregnancy dated?
Pregnancy is dated from the first day of the last menstrual period (LMP), not from conception. This means weeks 1–2 of pregnancy occur before conception. The due date is calculated as 40 weeks from LMP.
What is a full-term pregnancy?
Full term is defined as 39–40 weeks. Early term is 37–38 weeks; late term is 41 weeks; post-term is 42+ weeks.