Pregnancy

Pregnancy is the physiological state extending from fertilisation through to delivery. It involves coordinated maternal physiological adaptation across virtually every organ system to support fetal growth and development.

It includes the structured framework of antenatal care, which monitors this process and identifies and addresses any deviation from the expected, normal course.

This entry provides the overarching physiological and antenatal care framework underpinning the more detailed, condition-specific entries addressed under high risk pregnancies and the numerous individual pregnancy-related conditions addressed throughout this Atlas.

Physiological adaptation to pregnancy

• Cardiovascular adaptation comprises a substantial, progressive increase in cardiac output, reflecting increases in both stroke volume and heart rate. Systemic vascular resistance falls and, consequently, so does blood pressure during the first and second trimesters. Blood pressure typically returns toward pre-pregnancy levels in the third trimester.
• Circulating blood volume increases substantially. These coordinated changes are essential to supporting adequate uteroplacental perfusion throughout pregnancy.
• Respiratory adaptation comprises increased tidal volume and minute ventilation, driven predominantly by the respiratory stimulant effect of progesterone. This produces a mild, physiological respiratory alkalosis.
• The diaphragm elevates as pregnancy progresses. Vital capacity is generally preserved or even slightly increased, given compensatory changes in chest wall configuration.
• Haematological adaptation comprises a substantial increase in plasma volume, proportionally exceeding the increase in red cell mass. This produces the characteristic, physiological dilutional anaemia of pregnancy.
• Numerous clotting factors increase. This contributes to the physiological hypercoagulable state of pregnancy and its relevance to venous thromboembolism risk.
• Renal adaptation comprises a substantial increase in renal blood flow and glomerular filtration rate. The renal collecting system dilates.
• This physiological hydronephrosis of pregnancy is more pronounced on the right side, reflecting the anatomical relationship with the sigmoid colon and the direction of uterine dextrorotation.
• These renal changes affect the interpretation of renal function tests during pregnancy. They also increase susceptibility to ascending urinary tract infection.
• Endocrine and metabolic adaptation comprises substantial changes across numerous hormonal axes. Thyroid binding globulin increases, which needs specific consideration when interpreting thyroid function tests during pregnancy.
• Insulin resistance progresses as pregnancy advances. It is driven predominantly by placental hormone production and is directly relevant to the pathophysiology of gestational diabetes.
• Oestrogen and progesterone production increase substantially, predominantly from the placenta as pregnancy progresses beyond the early first trimester.

• Common, generally benign symptoms of normal pregnancy include nausea and vomiting, fatigue, breast tenderness, and urinary frequency. Nausea and vomiting are particularly prominent in the first trimester, with a more severe form, hyperemesis gravidarum.
• Urinary frequency reflects the combined effect of increased renal blood flow and, as pregnancy advances, mechanical bladder compression.
• As pregnancy progresses, the mechanical and hormonal effects of the enlarging uterus cause backache and symphysis pubis discomfort. Breathlessness, particularly in the third trimester, reflects diaphragmatic elevation.
• Structured antenatal care comprises a series of scheduled visits at defined gestational intervals. Each incorporates clinical assessment (blood pressure, symphysis-fundal height measurement, and urinalysis).
• It also includes structured screening (including combined first-trimester screening for chromosomal abnormality, and screening for gestational diabetes) and full anomaly ultrasound scanning.
• Antenatal care includes ongoing monitoring for any pregnancy-specific complication, addressed under the relevant specific conditions throughout this Atlas.
• Fetal growth and wellbeing assessment is an essential, ongoing component of antenatal surveillance throughout pregnancy. It incorporates serial symphysis-fundal height measurement and, where clinically indicated, growth ultrasound assessment.
• It also includes assessment of fetal movement. Maternal perception of reduced fetal movement is an important symptom warranting prompt further assessment, given its potential association with underlying fetal compromise.

• Pregnancy diagnosis is established through urinary or serum beta-human chorionic gonadotropin measurement. Once gestational age is sufficiently advanced, ultrasound confirms intrauterine pregnancy location and viability.
• Gestational age dating is an essential, foundational component of antenatal care. It directly informs the timing of subsequent screening tests, growth assessment, and, delivery timing planning.
• Dating is ideally established by first-trimester ultrasound measurement (crown-rump length). This is more accurate than dating by last menstrual period alone, particularly given the common occurrence of irregular menstrual cycles.
• Booking assessment at initial antenatal presentation is the essential foundation for individualised, risk-appropriate antenatal care planning.
• It incorporates detailed medical, obstetric, and social history, and baseline blood tests. These include full blood count, blood group and antibody screening, and screening for relevant infections including HIV, hepatitis B, and syphilis.
• It also includes risk assessment for the range of relevant pregnancy complications.

• Structured, risk-appropriate antenatal care is the essential framework for pregnancy management. Follow established national or regional guidance on the schedule and content of antenatal visits. Increase the intensity of monitoring according to individually identified risk factors.
• Lifestyle and health promotion advice is an essential, foundational component of antenatal care. It includes folic acid supplementation, smoking and alcohol cessation support, and general dietary and activity guidance.
• Folic acid is particularly important and ideally commenced before conception, given its well established relevance to neural tube defect risk reduction.
• Management of any identified pregnancy complication or relevant maternal medical condition follows the detailed, specific principles addressed under the numerous individual conditions throughout this Atlas.
• Structured, appropriately escalated multidisciplinary care is essential for pregnancies carrying elevated risk based on maternal, fetal, or pregnancy-specific factors.
• Birth planning incorporates discussion of delivery options and preferences, and individualised risk assessment informing the recommended mode and setting for delivery.
• It also includes preparation for labour, delivery, and the postpartum period. This underpins holistic management of this fundamental physiological process throughout its full course.