4.1 Antenatal Care Schedule, Physiological Adaptations & Routine Investigations

Antenatal Care Schedule, Physiological Adaptations & Routine Investigations

Universal antenatal care is the cornerstone of safe, preventive maternity practice. Guided by the National Institute for Health and Care Excellence (NICE) guideline NG201 (Antenatal care), the overarching aim of antenatal care is to optimize maternal and fetal health, foster an empowering therapeutic partnership, facilitate informed decision-making, and detect emerging medical, obstetric, or psychosocial complications early.

Midwives are lead professionals for women with uncomplicated pregnancies, acting autonomously to coordinate care while remaining vigilant for risk factors that necessitate multidisciplinary escalation.

---

The NICE Antenatal Appointment Schedule (NG201)

NICE NG201 establishes a differentiated schedule of routine antenatal visits based on parity for women with uncomplicated, singleton pregnancies. Nulliparous women require more frequent visits due to their higher baseline risk of undetected hypertensive disorders of pregnancy, fetal growth restriction, and labour dystocia.

---

Maternal Physiological Adaptations to Pregnancy

Pregnancy elicits profound, reversible physiological adaptations across every maternal organ system. These adaptations are orchestrated by placental steroid hormones (oestrogen, progesterone), human chorionic gonadotropin (hCG), human placental lactogen (hPL), and vasoactive peptides (relaxin, nitric oxide, prostacyclin).

1. Cardiovascular System

The cardiovascular system undergoes dramatic remodelling to satisfy the metabolic demands of the growing fetoplacental unit:

• Plasma Volume Expansion: Circulating plasma volume increases by approximately 45% (1,200–1,500 mL) above non-pregnant levels, beginning as early as 6 weeks and peaking at 32–34 weeks.
• Red Blood Cell Mass: Erythrocyte volume increases by only 20% to 30% under the stimulation of erythropoietin. Because plasma expansion outstrips the increase in red cell mass, a state of physiological haemodilution ensues, historically termed the physiological anaemia of pregnancy. This lowers maternal blood viscosity, decreasing peripheral vascular resistance and facilitating uteroplacental perfusion.
• Cardiac Output (CO): Cardiac output rises by 30% to 50%, driven initially by an increased stroke volume (up 20–30%) and subsequently by an elevation in resting maternal heart rate of 15 to 20 beats per minute (bpm).
• Systemic Vascular Resistance (SVR) & Blood Pressure: Progesterone, nitric oxide, and vascular endothelial prostacyclin stimulate profound vascular smooth muscle relaxation, decreasing SVR by up to 30%. Consequently, maternal blood pressure declines in early pregnancy, reaching its nadir between 20 and 24 weeks' gestation (with diastolic BP dropping by 10–15 mmHg), before gradually returning to pre-pregnancy baseline levels near term.
• Aortocaval Compression (Supine Hypotensive Syndrome): From mid-pregnancy onwards, the enlarged gravid uterus compresses the inferior vena cava and abdominal aorta when the woman lies supine. This causes an acute reduction in venous return, precipitously dropping stroke volume and maternal blood pressure, leading to maternal dizziness, pallor, syncope, and reduced uteroplacental perfusion. Midwives must ensure pregnant women are positioned semi-recumbent with a lateral tilt or in the left lateral position during assessments and labour.

2. Respiratory System

Respiratory adaptations prioritize maternal and fetal oxygenation and efficient carbon dioxide excretion:

• Progesterone Medullary Stimulation: Elevated progesterone directly stimulates the respiratory centre in the medulla oblongata, increasing its sensitivity to carbon dioxide.
• Ventilatory Changes: Tidal volume increases by 30% to 40% (from ~500 mL to ~700 mL), while the baseline respiratory rate remains essentially unchanged (12–20 breaths/min). As a result, maternal minute ventilation increases by 40% to 50%.
• Physiological Compensated Respiratory Alkalosis: Hyperventilation lowers maternal arterial carbon dioxide tension (PaCO2) to 3.7–4.2 kPa (28–32 mmHg) (non-pregnant baseline: 4.7–6.0 kPa / 35–45 mmHg), while arterial oxygen tension (PaO2) rises to 13–14 kPa (100–105 mmHg). This facilitates the diffusion gradient for carbon dioxide across the placenta from fetal to maternal circulation. Renal excretion of bicarbonate compensates for this hypocapnia, lowering serum bicarbonate to 18–22 mmol/L and preserving a physiological blood pH of 7.40–7.45.
• Diaphragmatic Elevation: The growing uterus elevates the diaphragm by up to 4 cm, reducing functional residual capacity (FRC) by 20%, which decreases maternal oxygen reserves and accelerates maternal desaturation during apnoea or hypoventilation.

3. Renal System

Renal hemodynamics adapt rapidly to accommodate increased fluid turnover and metabolic waste clearance:

• Renal Blood Flow & Glomerular Filtration Rate (GFR): Renal blood flow and GFR increase by 50% above non-pregnant baselines by the end of the first trimester, mediated by relaxin-induced renal vasodilation.
• Serum Biomarkers: Enhanced filtration accelerates the clearance of nitrogenous wastes. Consequently, normal pregnancy baseline serum creatinine falls to 40–65 µmol/L (compared to non-pregnant 60–100 µmol/L), and blood urea falls to 2.5–4.5 mmol/L.

• Renal Tubular Threshold & Physiological Glycosuria: The marked increase in GFR exceeds the tubular maximum reabsorptive capacity (TmG) for glucose in many women. As a result, physiological glycosuria occurs in up to 50% of pregnant women without maternal hyperglycaemia. However, persistent or heavy glycosuria (≥2+ on dipstick) mandates screening for gestational diabetes mellitus (GDM).
• Anatomical Hydroureter & Stasis: Progesterone causes relaxation of ureteric smooth muscle, while the expanding uterus exerts mechanical compression at the pelvic brim, resulting in physiological hydronephrosis and hydroureter (more prominent on the right side due to dextrorotation of the uterus and cushioning of the left ureter by the sigmoid colon). This urinary stasis predisposes pregnant women to ascending infection.

4. Gastrointestinal System

• Smooth Muscle Relaxation: Progesterone decreases gastrointestinal smooth muscle motility and tone.
• Reflux & Delayed Emptying: Lower oesophageal sphincter (LOS) tone is decreased, predisposing women to gastro-oesophageal reflux (heartburn), exacerbated by upward displacement of the stomach by the gravid uterus.
• Prolonged Colonic Transit: Increased transit time through the large intestine enhances water and sodium reabsorption, frequently resulting in constipation and predisposing to haemorrhoids.
• Biliary Stasis: Incomplete gallbladder emptying and altered bile acid composition increase the risk of gallstone formation.

5. Haematological System & Hypercoagulability

Pregnancy is a state of compensated, physiological hypercoagulability, an evolutionary adaptation designed to mitigate catastrophic haemorrhage at placental separation during the third stage of labour:

• Procoagulant Elevation: Marked increases in circulating clotting factors, notably Fibrinogen (Factor I), which rises by 50% to 100% to reach 4.0–6.0 g/L (non-pregnant: 2.0–4.0 g/L). Factors VII, VIII, X, and von Willebrand factor also rise substantially.
• Anticoagulant Changes: Natural anticoagulant activity declines, marked by a reduction in Protein S activity and acquired resistance to activated protein C.
• Fibrinolysis Inhibition: Circulating levels of plasminogen activator inhibitor types 1 and 2 (PAI-1 and PAI-2, synthesized by the placenta) increase, blunting fibrinolytic breakdown.
• Clinical Impact: The relative risk of venous thromboembolism (VTE) is 4 to 5 times higher during pregnancy and increases to 20-fold higher during the initial 6 weeks postpartum. Midwives must conduct formal VTE risk assessments at booking, upon hospital admission, and postpartum using the RCOG Green-top Guideline No. 37a risk scoring tool.

---

Comprehensive Booking Assessment Protocols

The booking interview represents the most exhaustive evaluation of pregnancy. Midwives gather structured data to individualize the care pathway:

1. Obstetric & Gynecological History: Previous gestations, parity, previous pre-eclampsia, gestational diabetes, preterm birth (<37 weeks), low birth weight (<10th centile), stillbirth, neonatal death, recurrent miscarriage (≥3), uterine surgery, or previous caesarean sections.
2. Medical & Surgical History: Pre-existing hypertension, chronic kidney disease, diabetes mellitus, epilepsy, cardiac disease, autoimmune disorders, and surgical procedures.
3. Psychiatric History: Universal screening using the Whooley Questions and GAD-2 for antenatal depression and anxiety; inquiry regarding personal or family history of bipolar affective disorder, postpartum psychosis, or severe depressive illness.
4. Social Complexity & Safeguarding: Universal, private routine enquiry into domestic abuse, female genital mutilation (FGM), modern slavery, substance misuse, housing insecurity, and child protection history.
5. Baseline Observations: Blood pressure (calibrated sphygmomanometer with appropriate cuff size), maternal height, weight, Body Mass Index (BMI), and universal Carbon Monoxide (CO) breath testing (threshold ≥4 ppm indicates active tobacco smoke or toxic environmental exposure, prompting referral to stop smoking services).

---

Routine Booking Investigations & Laboratory Screening

NICE NG201 stipulates routine baseline screening bloods and urinalysis to detect subclinical conditions that threaten maternal and fetal outcomes.

1. Full Blood Count & Anaemia Screening

• Haemoglobin (Hb) Diagnostic Thresholds:
  • Booking / First Trimester: Hb < 110 g/L
  • 28 Weeks / Second & Third Trimesters: Hb < 105 g/L
  • Postpartum: Hb < 100 g/L
• Etiology & Laboratory Investigation: Iron deficiency accounts for >90% of pregnancy anaemias. Initial evaluation reviews red cell indices: low Mean Corpuscular Volume (MCV <80 fL) and low Mean Corpuscular Haemoglobin (MCH <27 pg) indicate microcytic hypochromic anaemia. If haemoglobinopathy is excluded, a serum ferritin level <30 µg/L confirms depleted iron stores.
• Clinical Management: First-line treatment consists of oral elemental iron: ferrous sulfate 200 mg, ferrous fumarate 210 mg, or ferrous gluconate daily. Women should take oral iron on an empty stomach with water or a vitamin C-rich beverage to maximize absorption, avoiding tea, coffee, dairy products, or antacids within 2 hours. Repeat FBC is indicated after 2–3 weeks to confirm an appropriate haematological response (expected rise in Hb ≥10 g/L). Intravenous iron (e.g. ferric derisomaltose) is considered from the second trimester onwards when oral iron is not tolerated, not absorbed or not effective, or when iron deficiency is confirmed late in pregnancy (after about 34 weeks) and rapid repletion is needed (British Society for Haematology).

2. Blood Group, Rhesus D Status & Atypical Red Cell Antibodies

• ABO & RhD Grouping: Determines maternal ABO status and whether the red blood cells express the Rhesus D (RhD) surface antigen.
• Red Cell Atypical Antibody Screening: Screens maternal plasma for clinically significant alloantibodies (e.g. anti-D, anti-c, anti-Kell) capable of crossing the placenta and binding to fetal erythrocytes, causing haemolytic disease of the fetus and newborn (HDFN).
• Retesting: Antibody screening is performed universally at booking and repeated at 28 weeks' gestation.
• Prophylaxis: RhD-negative, non-sensitized women are offered Routine Antenatal Anti-D Prophylaxis (RAADP) at 28 weeks (either a single dose of 1500 IU or a two-dose regimen of 500 IU at 28 and 34 weeks), or non-invasive prenatal testing (NIPT) for fetal RHD genotype via cell-free fetal DNA to target anti-D only to women carrying an RhD-positive fetus.

3. Infectious Diseases Screening (Opt-Out Model)

Under UK NSC guidance, pregnant women are offered universal screening for three blood-borne infections on an opt-out basis:

1. Human Immunodeficiency Virus (HIV-1 and HIV-2): Identifying maternal HIV enables immediate combination antiretroviral therapy (cART), viral load suppression to <50 copies/mL, avoidance of breastfeeding or supported formula feeding, and neonatal post-exposure prophylaxis, reducing vertical transmission rates from >25% to <0.5%.
2. Hepatitis B Virus (HBV - HBsAg): Maternal surface antigen testing identifies carrier status. Neonates born to HBsAg-positive mothers receive immediate post-exposure prophylaxis: Hepatitis B vaccine within 24 hours of birth, plus Hepatitis B Immunoglobulin (HBIG) for higher-risk babies: the mother is HBeAg-positive, HBsAg-positive without e-markers, had acute hepatitis B in pregnancy, or has HBV DNA of 1 × 10⁶ IU/mL or more, or the baby weighs 1,500 g or less (Green Book). The baby then completes the childhood vaccine series to prevent chronic neonatal hepatitis and cirrhosis.
3. Syphilis (Treponema pallidum): Serological screening prevents congenital syphilis, intrauterine fetal demise, hydrops fetalis, and severe neonatal skeletal and neurological deformities. Maternal treatment with intramuscular benzathine penicillin G is curative.

4. Haemoglobinopathy Screening

• Objective: Identifies carriers of sickle cell disorders, thalassaemias, and other clinically significant haemoglobin variants.
• Strategy: In high-prevalence areas, screening is offered universally. In low-prevalence areas, screening is targeted using the Family Origin Questionnaire (FOQ) to trace maternal and paternal ancestral origins over three generations.
• Laboratory Methodology: High-Performance Liquid Chromatography (HPLC) or capillary electrophoresis, alongside FBC indices (MCH <27 pg triggers immediate thalassaemia testing regardless of FOQ).
• Paternal Testing: If the pregnant woman is identified as a carrier, urgent testing of the biological father is arranged immediately, ideally before 10 weeks, to determine whether the fetus has a 25% (1 in 4) chance of inheriting a major haemoglobinopathy (e.g. sickle cell anaemia, beta-thalassaemia major), allowing timely counselling and diagnostic options.

5. Midstream Urine Culture for Asymptomatic Bacteriuria

• Definition: The presence of ≥10^5 Colony-Forming Units (CFU)/mL of a single bacterial species in a clean-catch midstream urine (MSU) specimen collected from a woman without dysuria, frequency, or systemic symptoms.
• Pathophysiology & Risks: Due to progesterone-mediated ureteric smooth muscle dilatation, decreased bladder tone, and altered urine composition (glycosuria, aminoaciduria), asymptomatic bacteriuria (ASB) occurs in 2% to 10% of pregnancies. If left untreated, 20% to 40% of women with ASB develop acute maternal pyelonephritis, a severe systemic infection associated with maternal septic shock, acute respiratory distress syndrome (ARDS), preterm labour, and low birth weight.
• Treatment & Follow-Up: NICE NG201 mandates that ASB must be treated with a 7-day course of targeted oral antibiotics guided by microbial sensitivities (e.g. nitrofurantoin [avoided at full term due to risk of neonatal haemolysis], amoxicillin, or cefalexin). A repeat MSU culture is required 7 days after completing the antibiotic course as a formal test of cure.