TABLE 1.

Cumulative deficit in blood flow seen by the intervillous space for different percentages of the night spent sleeping supine throughout the third trimester, with corresponding percentage of total blood flow to the intervillous space that each cumulative deficit represents.

Proportion of night spent sleeping supine	Proportion uterine artery flow is reduced when supine a	Liters of blood per minute perfusing the uterus b	Proportion of uterine flow perfusing the placenta c	Number of minutes spent sleeping in third trimester d	Cumulative deficit
					Liters	Percentage of total blood flow to intervillous space e (%)
0.27 f	0.24	0.60	0.80	40,320	1,254	2.2
0.33 g					1,532	2.6
0.48 h					2,230	3.8

^aThere is a 24% reduction in uterine artery flow when supine in the third trimester. (Couper et al., 2021).

^bIn the third trimester, an average of 0.6 L of blood per minute perfuses the uterus. (Cunningham et al., 2023).

^cEighty percent of the uterine flow perfuses the placenta (while 20% perfuses the myometrium). (Cunningham et al., 2023).

^dAssuming 8 hours per day are spent sleeping (or resting in bed) from 28 through 40 weeks gestation.

^eTotal blood flow to the intervillous space is calculated as: 0.60 L/min * 0.80 * 60 min/h * 24 h/day * 7 days/week * 12 weeks.

^fThe average amount of the night spent supine in the third trimester in studies where sleeping posture was confirmed by continuous overnight video recordings is 27%. (Maasilta et al., 2001; Wilson et al., 2011; OBrien and Warland, 2014; Warland and Dorrian, 2014; McIntyre et al., 2016; Warland et al., 2018a; Kember et al., 2018; Dunietz et al., 2020; Wilson et al., 2022).

^gThe average percent of the night spent supine in the third trimester if the waking-up posture in the morning was supine is 33%. (Wilson et al., 2022).

^hThe average percent of the night spent supine in the third trimester if the going-to-sleep posture was supine is 48%. (Wilson et al., 2022).