Foetal development begins at the moment of conception, when a single spermatozoon penetrates a secondary oocyte, usually within the ampulla of the uterine tube. This union restores the diploid number of 46 chromosomes, creating a zygote. Within hours, the zygote undergoes rapid mitotic divisions known as cleavage while traveling down the fallopian tube toward the uterine cavity.
Implantation into the vascularized endometrium occurs roughly six to seven days after fertilization. During this period, the syncytiotrophoblast erodes into the maternal decidua, establishing the physical connection necessary for nutritional exchange and hormone production, particularly human chorionic gonadotropin.
The embryonic period spans from implantation through the eighth week of gestation. This phase is characterized by gastrulation, where the inner cell mass organizes into three distinct primary germ layers: the ectoderm, mesoderm, and endoderm. Each layer gives rise to specific organ systems.
Organogenesis—the creation of internal organs—occurs rapidly during weeks four to eight. Because foundational structures and organ systems are forming simultaneously, the embryo is exceptionally vulnerable to external insults, teratogens, and genetic disruptions during this tight window.
Following the embryonic period, the foetal stage extends from week nine until birth. Growth during this phase is primarily marked by rapid weight gain, cellular maturation, and functional refinement of established organ systems.
During the second trimester, quickening occurs as the mother feels foetal movements. The skin is covered by vernix caseosa and lanugo to protect against amniotic fluid. Alveoli begin to develop in the lungs, though gas exchange remains limited if delivery occurs too early.
The third trimester focuses heavily on fat deposition, central nervous system maturation, and surfactant production in the lungs. Subcutaneous fat smooths out the wrinkly skin and prepares the neonate for thermal regulation outside the womb. Iron stores are actively transferred from the maternal circulation during the final weeks.
Support structures outside the embryo ensure survival, metabolic exchange, and physical protection inside the amniotic sac.
| Structure | Primary Function |
|---|---|
| Placenta | Gas exchange, nutrient transport, and hormone synthesis |
| Amniotic Fluid | Cushions the foetus, allows movement, and prevents compression |
| Umbilical Cord | Connects the foetus to the placenta via two arteries and one vein |
The placenta acts as an endocrine organ, secreting progesterone and human placental lactogen to maintain pregnancy. Amniotic fluid is continuously swallowed and produced via foetal urination, keeping fluid volume stable. Abnormalities in fluid volume, such as oligohydramnios or polyhydramnios, often signal underlying renal or gastrointestinal anomalies.
Foetal circulation differs fundamentally from postnatal circulation because the lungs and liver are non-functional in utero. Oxygenated blood returns from the placenta through the umbilical vein, bypassing the immature liver via the ductus venosus to enter the inferior vena cava.
To bypass the fluid-filled lungs, blood takes two special shortcuts:
At birth, clamping the umbilical cord and the first neonatal breath drastically drop pulmonary vascular resistance, closing these shunts functionally and anatomically over time.
Midwives and nurses use several clinical tools to monitor foetal health and development throughout pregnancy. Routine antenatal visits track symphysis-fundal height to match gestational age in weeks, serving as a simple screening tool for intrauterine growth restriction.
A biophysical profile combines non-stress test results with ultrasound observations of foetal breathing movements, body movements, muscle tone, and amniotic fluid volume to grade overall well-being.
A teratogen is any environmental agent that causes structural or functional abnormalities in a developing embryo or foetus. The impact of a teratogen depends on the specific agent, dosage, route of exposure, genetic susceptibility, and exact gestational age at the time of exposure.
Common teratogens include:
Maternal nutritional status also plays a significant role; for instance, adequate folic acid intake prevents neural tube defects, while severe maternal malnutrition can impair overall cellular growth.