A large meta-analysis released 4/2/19 showed a clinically significant increase in stillbirth for pregnant women who routinely went to sleep on their back compared to those who fell asleep on either of their sides. The effect of falling to sleep position was not significant until the 3^rd trimester. The study found a 2.6-fold increase in stillbirth in women who usually fell asleep on their backs in the last weeks of pregnancy. The researchers understand that women will sometimes find themselves on their backs during the night. The position in which a person initially falls asleep is the position they sleep in the longest. So going to sleep on their side has the biggest effect on the pregnancy. They recommend that should a woman wake up on her back, she not panic and just roll over onto either side at that point. The study looked at over 800 pregnancies that ended in stillbirth and compared to 2200 pregnancies that did not. The study identified several factors each independently associated with stillbirth including going to sleep on their back.

Modifiable risk factors for late term stillbirth
< 10th birthweight customised centile
Decreased fetal movements (last two weeks)
Obesity (BMI > 30 kg/m²)
Smoking (beyond first trimester)
Overweight (BMI 25–29.9 kg/m²)
Supine going-to-sleep position (last two weeks)

(Source) Stillbirth is a devastating outcome of pregnancy that happens in approximately 1% of all pregnancies. Any information that can reduce this risk is valuable.

Medical Conditions Associated with an Increased Risk of Stillbirth Include:

Hypertension disorders
Diabetes
Maternal obesity

Pregnancy Risk Factors Include:

Multiple gestation
Intrahepatic Cholestasis of Pregnancy (ICP)
First baby or >5^th baby
Poor outcomes in prior pregnancies
History of miscarriage or stillbirth in prior pregnancy

(Source) The exact cause of stillbirth is not completely known because, thankfully, even in the presence of these risk factors most babies are born alive and well. Based on this information, we recommend that after 28 weeks of pregnancy, you go to sleep on one side or the other.