Antenatal care includes a wide range of functions, including:
• Provision of information to mothers about the importance of diet and lifestyle, and about choices available to them during their pregnancy.
• Screening for pre-existing maternal conditions and risk factors, including those that may influence the anaesthetic care of the woman, and the construction of an individualised peripartum management plan.
• Assessment of maternal wellbeing during pregnancy and screening for development of pregnancy-specific and non-specific conditions.
Maternal assessment
Specific maternal conditions and their assessment and management are addressed in subsequent chapters. For routine antenatal care in the UK, ten antenatal appointments are currently recommended for nullipara with uncomplicated pregnancies, and seven for multipara, with regular urine and blood pressure checks and further checks (e.g. for anaemia) at specified intervals. This may be midwifery- or obstetrician-led, depending on the progress of the pregnancy, previous complications or the presence of comorbidities. Anaesthetic involvement may be ad hoc or via formal clinics, in which anaesthetists can assess women referred to them and arrange further referral or investigations and discuss management options. A checklist for midwives and obstetricians, listing those conditions that warrant anaesthetic referral, is useful (Table 12.1).
Fetal assessment
It is possible to make detailed assessments of fetal wellbeing in the antenatal period that allow monitoring of fetal growth and development, and enable the detection of fetal hypoxia. A decision to deliver the baby early may be made on the outcome of these assessments, and the obstetric anaesthetist may be involved in this decision making. The most commonly used methods of antenatal assessment are symphysis-fundal height measurement, serial ultrasonography, serial Doppler flow studies and cardiotocography.
• Symphysis-fundal height measurement should be performed from 24 weeks of pregnancy and may enable the prediction of a small-for-gestational-age (SGA) fetus. Measurements obtained are plotted on customised growth charts, and a single value below the 10th centile or multiple ones suggesting a slow rate of growth may trigger further assessment by ultrasound.
Table 12.1 Checklist for antenatalanaesthetic referral
|
Obstetric |
Placenta accreta/percreta Three or more fetuses |
|
Cardiovascular/respiratory/neurological |
All except mild, without functional limitation, and without history of previous surgery |
|
Haematological |
Coagulopathy, including anticoagulant therapy Sickle cell variants Thrombocytopenia |
|
Musculoskeletal |
Severe back or neck problems or previous surgery Spina bifida Scoliosis |
|
Anaesthetic |
Previous difficulties, e.g. intubation or failed or poor regional analgesia or anaesthesia Family history of malignant hyperthermia or suxamethonium apnoea |
|
Other |
Severe obesity Severe allergy to drugs or latex Severe anxiety/needle phobia Severe connective tissue disease Poor venous access, e.g. intravenous drug use Refusal of blood products Inability to give consent Language/communication barrier |
• Serial ultrasonography. Routine scanning is offered at 10-13 weeks and 18-20 weeks. The head circumference is measured in association with the abdominal circumference.
If the fetus is starving, glycogen stores in the liver will be depleted and there will be an increase in the ratio of head circumference to abdominal wall circumference (asymmetrical growth restriction). There may also be a generalised growth restriction (symmetrical growth restriction). The liquor volume is also used as an indicator of fetal wellbeing and placental function, poor placental function being reflected in a reduced liquor volume.
As a measure of this volume, the anterior-posterior distance across the liquor (the liquor column) is measured using a transducer. A column of less than 3 cm is indicative of oligohydramnios and one less than 2 cm represents very severe oligohydramnios.
The amniotic fluid index may also be used to measure the liquor volume; this is the sum of the liquor column in each of the four liquor quadrants and is normally 8-20 cm.
• Serial Doppler flow studies. Both maternal uterine blood flow and fetal umbilical artery blood flow may be measured using Doppler techniques. The pattern of flow is influenced by resistance of the placental vasculature and reflects placental function as follows:
1. Normal flow continues through systole and diastole, as there is little resistance to flow through the placenta. The systolic/diastolic flow velocity ratio (SD ratio) is widely used to indicate resistance to arterial flow; several other derived indices (e.g. pulsatility index) have also been used to indicate fetal perfusion and oxygenation. The use of these techniques for screening for high-risk fetuses is controversial, and they may be reserved for monitoring known high-risk cases.
2. Just absent end-diastolic flow may indicate the need for delivery of the baby. Wide absence of end-diastolic flow suggests the need to deliver the baby urgently.
3. Reversal of end-diastolic flow suggests the need for immediate delivery of the baby. Plans for timing and mode of delivery may be based on the evidence of the Doppler studies. The anaesthetist should understand that the anaesthetic management should optimise the placental flow, and that meticulous care should be taken to avoid sudden cardiovascular changes and in particular supine hypotension.
In specialised fetal medicine units, blood flow may also be measured by using Doppler techniques in the fetal abdominal aorta, renal or middle cerebral arteries.
• Cardiotocography. This will only record the fetal heart during the time of the trace and cannot provide historical or predictive information. The pattern of the trace may be indicative of fetal compromise and may be used to plan the mode of delivery, e.g. either induction of labour or caesarean section (see Chapter 20, Intrapartum fetal monitoring). It is important that the anaesthetist communicates with the obstetrician and understands how compromised the fetus is when asked to give analgesia or anaesthesia to these mothers. The degree of urgency for the delivery will depend on the condition of the fetus. It should be remembered that women in these circumstances may be very anxious and upset and will need extra support during delivery.
Key points
• Antenatal assessments may identify mothers and fetuses at special risk.
• High-risk mothers should be referred for antenatal anaesthetic assessment.
• Meticulous care should be taken to maintain optimal placental perfusion if investigations indicate fetal hypoxaemia.
• Communication between medical and midwifery staff is crucial.
Further reading
Everett TR, Peebles DM. Antenatal tests of fetal wellbeing. Semin Fetal Neonatal Med 2015; 20: 138-43.
Grivell RM, Alfirevic Z, Gyte GM, Devane D. Antenatal cardiotocography for fetal assessment. Cochrane Database SystRev 2015; (9): CD007863.
National Institute for Health and Care Excellence. Antenatal Care for Uncomplicated Pregnancies. Clinical Guideline 62. London: NICE, 2008 (updated 2017). www.nice.org.uk/guidance/CG62 (accessed December 2018).