Aortocaval compression can be avoided by lying in the full left lateral position. Unfortunately, it is not practical to carry out obstetric procedures in this position, so it is usual to modify the supine position by tilting the mother towards her left side. A tilt of at least 15 degrees is recommended to reliably avoid aortocaval compression.

This is quite an extreme position, and there is a risk of the mother sliding off the bed if steps to prevent that aren’t taken. In practice, anaesthetists often use much less tilt than the recommended 15 degrees, and this is likely to be an important contributory factor in the aetiology of hypotension during spinal anaesthesia in obstetrics.

Left lateral tilt is required from 20 weeks of pregnancy onwards, and is sometimes necessary earlier than this.