Pregnancy is the physiological state in which an embryo or fetus develops within the uterus following conception and implantation, lasting approximately 40 weeks when measured from the first day of the last menstrual period, or approximately 38 weeks from fertilisation. It involves profound anatomical, hormonal, metabolic, cardiovascular, respiratory and immune adaptations in the pregnant person's body. Pregnancy is conventionally divided into three trimesters, although clinically important developmental milestones occur continuously throughout gestation.
The process begins with fertilisation and implantation and progresses through embryonic development, fetal growth, maturation of organ systems and preparation for birth. Pregnancy is a normal physiological state rather than an illness, but it creates changing physiological demands and can coexist with medical or obstetric complications requiring specialised care.
Early Pregnancy involves fertilisation, movement of the fertilised egg through the fallopian tube and implantation into the uterine lining. The developing conceptus produces signals that support continuation of the pregnancy, including production of human chorionic gonadotrophin, which is detected by pregnancy tests and helps maintain the hormonal environment needed for early gestation. The placenta subsequently develops as a specialised interface between maternal and fetal circulations, enabling exchange of oxygen, nutrients and waste products while also functioning as an endocrine organ. Embryonic development is particularly sensitive during early gestation because major organ structures are established during this period. The timing of developmental events explains why exposure to certain medications, infections, toxins or nutritional deficiencies can have different consequences depending on when exposure occurs.
Maternal physiology changes progressively throughout Pregnancy. Blood volume increases, cardiac output rises and the cardiovascular system adapts to increased circulatory demands. Respiratory physiology also changes, with increased ventilation and altered perception of breathlessness in some individuals. Hormonal changes affect the gastrointestinal tract, kidneys, skin, breasts and musculoskeletal system, while the growing uterus progressively changes abdominal and pelvic anatomy. These adaptations can produce normal symptoms such as nausea, fatigue, urinary frequency, breast tenderness, reflux and changes in appetite. Distinguishing expected physiological changes from pathological symptoms is a central part of antenatal assessment because normal pregnancy can resemble disease in several respects.
Fetal development progresses from early cellular differentiation to formation and maturation of organ systems. Growth is influenced by genetic factors, placental function, maternal health, nutrition, oxygenation and other environmental conditions. The placenta performs transport, endocrine and immunological functions and is essential for maintaining the pregnancy, while the umbilical cord connects the fetus to the placental circulation. Fetal growth can be assessed through clinical measurements and ultrasound when indicated, although estimates are not perfectly precise. Reduced fetal growth may result from placental insufficiency or other causes, whereas excessive growth can also have clinical implications. Monitoring therefore focuses not only on gestational age but also on growth trajectory and evidence of fetal wellbeing.
Pregnancy is accompanied by changes in risk assessment and medical decision-making. Some pre-existing conditions, including hypertension, diabetes, thyroid disorders, epilepsy and cardiovascular disease, may require altered management during pregnancy because physiological changes can affect disease control and medication choices. Certain complications arise specifically because of pregnancy, while others represent conditions that happen coincidentally during pregnancy. Medication decisions require assessment of maternal benefit, fetal exposure and available safety evidence rather than assuming that all medicines are either completely safe or absolutely prohibited. Similarly, symptoms such as abdominal pain, bleeding, severe headache, visual disturbance, breathlessness or reduced fetal movement may have different implications depending on gestational age and associated findings.
Pregnancy culminates in labour, birth and delivery of the placenta, although the timing and course vary substantially. Labour involves coordinated uterine contractions, cervical changes and descent of the fetus through the birth canal in vaginal birth, while caesarean birth involves surgical delivery through abdominal and uterine incisions. Decisions about mode of birth depend on obstetric circumstances, fetal position, placental location, previous surgical history, maternal and fetal health and other factors. After birth, major physiological changes occur as placental hormone production ends and the body transitions towards the postpartum state. Pregnancy is therefore not an isolated nine-month event but part of a continuous reproductive process extending from conception through birth and postpartum recovery.
The historical understanding of Pregnancy has developed from empirical observations of fertility, fetal development and childbirth into a field supported by endocrinology, embryology, genetics, ultrasound, fetal medicine and evidence-based obstetrics. Modern pregnancy care can identify many developmental and maternal conditions earlier than was previously possible, but no monitoring system can predict or prevent every complication. A common misconception is that pregnancy should always feel uniformly healthy or that every symptom is expected, whereas clinically important abnormalities can occur within an otherwise uncomplicated pregnancy. Another misconception is that pregnancy milestones occur on identical dates for everyone; gestational development follows ranges and individual variation.
Pregnancy has major biological, social and healthcare significance and requires an approach that recognises both its normal physiological nature and the possibility of complications. Appropriate care involves monitoring maternal wellbeing, fetal development and pregnancy-specific risks, while also supporting informed decisions about nutrition, screening, medicines, activity and birth planning. The pregnant person's symptoms, medical history and reproductive circumstances influence what constitutes appropriate assessment. Pregnancy also illustrates the interdependence between maternal and fetal physiology: placental function, maternal circulation, nutrition and oxygenation directly influence the developing fetus. Understanding these relationships is essential for interpreting antenatal findings and distinguishing normal adaptation from conditions requiring intervention.
