The ALS manual states that after 20 weeks gestation the pregnant uterus can cause compression of the inferior vena cava (IVC). The rationale for this is as follows. The uterus normally leaves the confines of the pelvis at the end of the 1st trimester (approx 13 weeks). At this time it may be palpable abdominally but not visible. The uterus reaches the umbilicus at approximately 20 weeks at which point it is usually visible. IVC compression causing symptoms is rarely reported before 20 weeks (although pressure in the femoral veins can be elevated in the supine position from 16 weeks). The IVC bifurcates at approximately L4/5. It is unlikely that caval compression will occur until the uterus has reached this height which is beyond the pelvic brim. Clearly the exact timing is variable and parturients with multiple pregnancies (twins, triplets) and those with an excess of amniotic fluid (polyhydramnios) will experience symptoms of IVC compression earlier than those with small singleton pregnancies. Some women hardly look pregnant at full term whilst others appear to have a term pregnancy early in their 3rd trimester. Furthermore, the reduction in cardiac output in the supine position will depend on the effectiveness of the collateral venous return through the azygous and ovarian veins - this varies hugely between patients.
This means there needs to be a pragmatic and simple approach when giving advice concerning the necessity of avoiding IVC compression during attempts at CPR in the pregnant patient. If the women is known to be 20 weeks pregnant or more, then IVC compression may occur and it should be relieved either by manually displacing the uterus to the left (pulling or pushing depending on the side of the patient) with one or two hands (keeping the chest supine) or by tilting the mother (head to toe) to the left (15-30 degrees) provided she is on a firm surface (e.g. tilting table or spinal board), so that the efficacy of chest compressions is not compromised. If the gestational age of the mother is unknown and the uterus is clearly visible then IVC compression may occur and uterine displacement (see above) is necessary. If the gestational age is not known and the mother is obese i.e. the uterus not necessarily visible, but it can be palpated at the level of the umbilicus then again IVC compression is possible and uterine displacement should be attempted. If the mother is known to be less than 20 weeks pregnant BUT (because of polyhydramnios or multiple pregnancies) the uterus is clearly visible and/or palpable at the level of the umbilicus, then IVC compression may still occur and uterine displacement should once again be attempted. Clearly if the mother is known to be pregnant but no uterus can be seen or felt (or if palpable, does not reach the umbilicus) then IVC compression is highly unlikely and no displacement manoeuvres need be attempted.