Pregnancy

First Trimester of Pregnancy (Weeks 1–12)

  • Marks the initial 12 weeks of pregnancy and includes the most critical stages of early development.

  • For the first six weeks post-fertilization (about eight weeks from the last menstrual period – LMP), the developing human is called an embryo.
    After this, it is referred to as a     fetus.

  • Begins with fertilization, followed by:

    • Implantation of the blastocyst in the uterus

    • Placenta formation

    • Organogenesis (development of major organ systems)

  • The embryo is highly susceptible to environmental influences
    (e.g., drugs, infections, radiation), making this the most vulnerable period for birth defects and developmental disorders.

  • The risk of miscarriage is highest in this trimester, especially due to chromosomal abnormalities or implantation failure.

By the end of the first trimester:

  • The heart begins to beat (around week 6)

  • The brain, spinal cord, limbs, and facial features start forming

  • All major organ systems are present in rudimentary form

  • The embryo becomes a fetus (after 8 weeks)

Hormonal regulation:

  • hCG, progesterone, and estrogen:

    • Support the pregnancy

    • Maintain the uterine lining

    • Prevent menstruation

Second Trimester of Pregnancy (Weeks 13–26)

  • Often called the “golden period” of pregnancy due to reduced morning sickness and increased maternal energy.

  • The fetus grows rapidly in size and weight, with accelerated growth compared to the first trimester.

  • The placenta is fully developed and functional, supplying oxygen and nutrients, removing waste.

  • Fetal movements (quickening) are typically first felt by the mother around weeks 18–20.

  • The sex of the baby can usually be identified by ultrasound around week 18.

  • The fetus develops fine hair (lanugo) and a waxy protective coating (vernix caseosa).

  • Organs continue to mature:

    • Lungs start forming respiratory structures.

    • Brain undergoes rapid development — the most important growth period.

    • Eyes begin to move and can sense light.

    • Ears start to hear external sounds.

  • The skeletal system hardens, facial features become more defined, and reflex responses and muscular activity develop.

⚠ Developmental Defects in the Second Trimester:

  • The highest risk of major congenital malformations is during the first trimester, but some developmental issues can still arise in the second trimester.

  • These include:

    • Neural tube defects (e.g., spina bifida), detectable via ultrasound.

    • Congenital heart defects.

    • Limb abnormalities.

    • Intrauterine Growth Restriction (IUGR) — growth restrictions.

Many of these can be detected during the anomaly scan (mid-pregnancy ultrasound) usually performed around 18–22 weeks.

Third Trimester of Pregnancy (Weeks 27–Birth)

  • Begins at week 27 and continues until the birth of the baby.

  • All major organ systems continue to grow and mature, including the brain, kidneys, and lungs.

  • The fetus gains most of its weight and develops fat to help regulate body temperature after birth.

  • Movements become stronger and more coordinated.

  • The uterus expands significantly, which may cause increased physical discomfort for the mother.

  • The fetus typically moves into a head-down position in preparation for delivery.

  • The fetus is considered full-term at 37 weeks or later.

  • This trimester ends with labor and birth, usually around week 40.