Embryology

What is it?

According to MedlinePlus: "Pregnancy (gestation) is the period between fertilisation and birth, during which the baby grows and develops inside the mother's womb". A typical pregnancy lasts from 37 to 42 weeks and consists of three trimesters.

The page lists the terms: zygote: the egg and sperm combined, carrying all the genetic information; blastocyst: a ball of cells that implants in the womb; embryo: the developing structure until the end of week 10; fetus: from week 10 until birth.

MedlinePlus's pregnancy topic page stresses that the body continues to change as the fetus grows until the baby is ready for birth, and that regular follow-up visits monitor the pregnancy. That page did not detail the stages of development; the detail is on the "Fetal development" page above.

Why is it done?

Knowing the weekly stages of development helps in understanding when the main organs form and when the fetus becomes more vulnerable to harmful factors, according to what MedlinePlus shows.

Key points

**Weeks 1–2:** the body prepares for pregnancy, and at the end of week 2 an egg is released from one of the ovaries.

**Week 3 (fertilisation):** the sperm meets the egg in the fallopian tube and the zygote forms, and divides into a blastocyst: a group of inner cells and an outer shell; the inner cells become the early embryo and the outer cells form protective membranes.

**Week 4 (implantation):** the blastocyst implants in the wall of the womb and is nourished from the mother's blood.

**Week 5:** the embryonic stage begins and "all the major organs and structures" form, and the brain, spinal cord, heart and digestive system begin to take shape. **Note from the page:** this is the period when the baby is most vulnerable to damage from factors that may cause birth defects, including some medicines, drugs, alcohol and infections.

**Weeks 6–7:** limb buds appear, the brain divides into five regions, the eyes and ears form, and the heart beats at a regular rhythm. **Week 8:** the limbs lengthen and the hands and feet look like little paddles and the lungs begin to form. **Week 9:** hair follicles and toes appear and the essential organs begin to grow.

**Week 10:** at its end the baby is no longer an early embryo but becomes a fetus, and facial features become distinct and the eyelids form. **Weeks 11–14:** the eyelids close and stay closed until week 28, and nails and tooth buds appear, and the head is large (about half the size of the fetus).

**Weeks 15–21:** the skin is almost transparent and fine downy hair (lanugo) appears, the bones harden, and movement and sucking movements begin. Hearing develops, and the mother feels the first movements (quickening) as a fluttering in the lower abdomen. **Week 22:** fine hair covers the body, meconium (the first stool) forms, eyebrows and eyelashes appear, and the heartbeat can be heard with the doctor's stethoscope.

**Weeks 23–30:** the bone marrow produces blood cells, the lower airways form, and fat storage begins. At week 26 the eyes are complete and fingerprints form, but the lungs are immature and not ready to work outside the womb. At weeks 27–30 the brain grows rapidly and the lungs produce surfactant.

**Weeks 31–40:** rapid growth and fat gain, rhythmic breathing, and the body stores iron, calcium and phosphorus. By weeks 35–37 the heart and blood vessels are complete, and weight approaches 5.5 pounds according to the page. At weeks 38–40 (completion) the fine hair mostly disappears and scalp hair thickens.

How is it done?

The page is descriptive and educational and does not give treatment recommendations. Weeks are determined by counting from the start of pregnancy; it states that at the 40th week of pregnancy 38 weeks have passed since fertilisation.

Risks and side effects

MedlinePlus cautions that the fetus at the start of the embryonic stage (from week 5) is the most vulnerable to damage from factors that may cause birth defects, such as some medicines, drugs, alcohol and infections. Discuss with your doctor any medicine or exposure during pregnancy or when planning it.

Limits of the test

The source of this page is MedlinePlus (last reviewed 8/18/2025). The page is not a substitute for pregnancy follow-up with the doctor, as development varies between cases, and detailed weight and length measurements are not mentioned here.

Common questions