Anatomical Changes & Foetal Development
Uterine Growth and Trimesters
A typical pregnancy lasts roughly 40 weeks, divided into three trimesters. Understanding the size and position of the uterus is critical for both fundal height assessment and trauma management.
- First Trimester (Weeks 1-12): Uterus is roughly the size of a grapefruit. It sits mostly protected within the bony pelvis but starts to emerge upwards by week 12.
- Second Trimester (Weeks 13-28): By week 20, the uterus weighs ~1kg and sits prominently within the abdomen (fundus usually at the umbilicus at 20 weeks).
- Third Trimester (Weeks 29-Birth): Roughly the size of a watermelon. Extends from the pubis symphysis up to the lower ribs, displacing abdominal organs upward and outward.
Systemic Physiological Adaptations
Pregnancy induces profound physiological changes designed to support foetal growth. These alter a woman's baseline vital signs and impact how she presents during medical emergencies or trauma.
1. Cardiovascular System
- Blood Volume: Total blood volume increases by 40-50% to perfuse the uterus and prepare for blood loss during birth.
- Physiological Anaemia: Plasma volume increases more than red blood cell mass. This dilutional effect causes a relative anaemia.
- Heart Rate: Resting HR increases by 15-20 beats per minute to manage the increased cardiac output.
- Blood Pressure: Systemic vascular resistance drops due to progesterone. BP often dips in the 2nd trimester but returns to baseline in the 3rd trimester.
2. Respiratory System
- Anatomy: The growing uterus pushes the diaphragm upward by ~4cm, decreasing Functional Residual Capacity (FRC).
- Ventilation: Progesterone acts as a respiratory stimulant. Tidal volume increases by up to 40%, increasing minute volume.
- Oxygen Demand: Maternal oxygen consumption increases by 20% to support the foetus and placenta.
3. Gastrointestinal System
- Motility: Progesterone relaxes smooth muscle, delaying gastric emptying and slowing intestinal motility (leading to constipation and nausea).
- Sphincter Tone: Relaxation of the lower oesophageal sphincter increases the risk of acid reflux.
4. Endocrine & Metabolic Systems
- Hormones: Massive increases in Estrogen, Progesterone, Human Chorionic Gonadotropin (hCG), and Relaxin.
- Metabolism: Basal Metabolic Rate (BMR) increases significantly.
- Insulin Resistance: Placental hormones cause maternal cells to become insulin resistant to ensure glucose remains in the bloodstream for foetal transfer. This can lead to Gestational Diabetes if the maternal pancreas cannot compensate.
5. Musculoskeletal System
- Ligament Laxity: The hormone relaxin softens pelvic joints and ligaments to prepare for birth, increasing the risk of sprains.
- Posture: The centre of gravity shifts forward, causing compensatory lumbar lordosis (exaggerated spinal curve) and frequent lower back pain.
Clinical Relevance & Presentation Differences
Masked Shock in Trauma
Clinical Correlation: Because a pregnant woman has up to 50% more blood volume, she can lose a significant amount of blood (up to 30-35%) before displaying classic signs of hypovolaemic shock (hypotension, severe tachycardia).
By the time maternal vital signs crash, foetal perfusion has likely been severely compromised for a long period, as the maternal body will aggressively shunt blood away from the uterus to preserve the mother's brain and heart.
Supine Hypotensive Syndrome & Airway Management
Cardiovascular Impact: In the 2nd and 3rd trimesters, laying a pregnant woman flat on her back causes the heavy uterus to compress the inferior vena cava (IVC). This drastically reduces venous return to the heart, slashing cardiac output and causing severe hypotension and syncope.
Management: Always tilt pregnant patients >20 weeks gestation to their Left Lateral side (15-30 degrees) to displace the uterus off the IVC. If performing CPR, manually displace the uterus to the left.
Respiratory/GI Impact: Due to decreased FRC (less oxygen reserve) and increased oxygen demand, pregnant patients desaturate incredibly fast if they stop breathing. Furthermore, delayed gastric emptying makes them a massive aspiration risk if they lose consciousness. Airway management must be aggressive and proactive.
Obstetric History Taking
Gravidity and Parity (G & P)
Understanding a woman's reproductive history is vital. Recent medical literature (Creinin & Simhan) highlights the importance of clear communication regarding these terms to distinguish between viable births and early losses.
| Term | Definition | Example |
|---|---|---|
| Gravidity (G) | The total number of times a woman has been pregnant, regardless of the outcome. This includes her current pregnancy. | A woman currently pregnant, who has had 1 miscarriage and 1 full-term birth is G3. |
| Parity (P) | The number of deliveries that have reached a potentially viable gestational age. Standard clinical utility suggests defining this as deliveries beginning at the 24th week of gestation, regardless of whether the baby was born alive or stillborn. | Using the example above, if her miscarriage was at 10 weeks, and her birth was at 39 weeks, she is P1. |
The 'CCLUE' Assessment
When assessing a late-stage pregnant patient, use the CCLUE acronym to gather specific obstetric information:
- C - Contractions: Are they present? Ask about frequency (how far apart) and duration (how long they last).
- C - Complications: Any known issues? (e.g., previous pre-term deliveries, previous Caesarean sections, placenta praevia, gestational diabetes).
- L - Leaking: Rupture of membranes (ROM/water breaking)? Bloody show? Any vaginal bleeding?
- U - Urge: Does she have an urge to push, or an urge to urinate/use her bowels? (A strong urge to use bowels often indicates the baby's head is descending into the birth canal).
- E - Expected Delivery Date: What is her EDD? (Calculates current gestational age).
Obstetric Handover
IMIST AMBO Adaptation
When delivering a handover for a pregnant patient at the hospital, standard structures like IMIST AMBO are slightly adapted to front-load vital obstetric information.
- Patient Name, Age, and Gender
- Obstetric Status: State the Gravidity (G), Parity (P), and Expected Delivery Date (EDD) / Gestational Age immediately.
- Presenting History: Incorporate your CCLUE findings here.
- Vital Signs (VSS): Remember to contextualize these against normal pregnant baseline physiology.
- Past History & Medications: Highlight specific pregnancy complications.
- Paramedic Provisional Diagnosis & Management
Crucial Addition: If a birth has occurred pre-hospitally, you must provide two distinct handovers: One for the mother, and a separate, comprehensive handover for the neonate (including time of birth, APGAR scores, and resuscitation efforts if any).