The consumption of alcohol during pregnancy has decreased drastically since it became known that it could cause Fetal Alcohol Spectrum Disorder (FASD)1, a neurodevelopmental condition. But surely just two glasses of wine can’t hurt during pregnancy, can it?

Before going ahead and drinking those glasses of wine, a pregnant woman should always check which trimester or stage of pregnancy she’s in. Why does it matter which trimester or stage a pregnant woman is in? What could those two glasses of wine possibly do? To answer these questions and more, we need to discuss sleep, particularly Rapid Eye Movement (REM) sleep, and what happens to the unborn child’s developing brain in the womb.

The developing brain of the foetus

Growing a brain is not a small ask; it’s possibly the most complex thing in the world. The cells in the brain are called neurons, and about 100 billion of them are sheltered under our bony skulls. Now, these neurons go on to form connections to each other, totalling about 100 trillion links or synapses, and it is this synapse formation, or synaptogenesis, that allows different parts of the brain to talk to each other. This process of connectivity is crucial for brain development, and this occurs during Rapid Eye Movement or REM sleep. In fact, to fully exploit the opportunity for synaptogenesis, the foetus will ramp up its consumption of REM sleep to almost nine hours a day. In the last week before birth, REM sleep hits a lifetime high of twelve hours per day. 

Disruption of REM sleep in rats

What happens if you disrupt REM sleep in a foetus or developing infant? Obviously, researchers couldn’t investigate this on human babies, so they began to study newborn rat pups. They found that when they deprived the infant rats of REM sleep, they stalled the building and development of the brain.2 Without REM sleep, the continued construction and development of the brain ground to a halt, like when a strike occurs in a company, the workers down their tools, and all productivity ceases. When deprived of REM sleep, the cerebral cortex demonstrated no change in growth. This effect also occurred when other researchers deprived different mammalian species of REM sleep, suggesting that synaptogenesis universally requires this particular stage of sleep. When the researchers allowed the infant rat pups to get some REM sleep, the construction and development of the cerebral cortex resumed, but the work was not accelerated to compensate for the lost Rapid Eye Movement sleep. An infant’s brain without REM sleep will have a brain that is not fully constructed.

Disruption of REM sleep in humans

A pregnant woman can achieve the same thing the scientists did to the rat pups with the two glasses of wine. Alcohol is one of the most potent suppressors of REM sleep that we’re aware of. 

When a pregnant woman consumes alcohol, it readily crosses the placental barrier, giving the developing foetus its first taste of this inebriating substance. Since researchers knew that alcohol readily crosses the placenta, they chose an extreme cohort of individuals to study the effects that alcohol has on the sleep of infants; they chose pregnant alcoholic women. As soon as the women gave birth, the researchers placed electrodes on the heads of the newborns to assess their sleep. What they found was that the newborns of heavy-drinking mothers spent far less time in REM sleep when compared to infants whose mothers did not consume any alcohol during their pregnancy.

The researchers also learned that both the quantity and quality of REM sleep were reduced, and the customary chaotic wave pattern of REM sleep that resembles a waking state was more subdued in the infants of alcoholic mothers.3

How do two glasses of wine affect the fetus?

Before we continue, just a word about the spelling of foetus, or is it fetus?  In most countries, fetus is the preferred spelling, while foetus is still used in the UK. From here on, we will use the spelling fetus because it’s easier to spell, and I want to give my Grammarly editor a rest.

Babies who were exposed to lots of alcohol before birth can have problems with their behaviour, how their brain works, the patterns of electrical activity in their brain, and how they sleep.4  Fortunately, few women drink excessively during their pregnancy, and scientists have developed methods to noninvasively track and monitor the heart rate in addition to the body, eye, and breathing movements of the unborn child in the womb. This has allowed scientists to determine the basic stages of NREM and REM sleep of the fetus in the womb. With the equipment in place, the researchers studied the sleep of babies just weeks from birth. The mothers were assessed on successive days whilst drinking non-alcoholic fluids on the first day, followed by the absolute minimum alcohol allowed, two glasses of wine. What they found was that those two glasses of wine significantly reduced the amount of time the unborn baby spent in REM sleep relative to when their mothers drank non-alcoholic fluids.

The research also revealed that the intensity of the REM sleep was dampened in the unborn babies when their mothers consumed alcohol. The researchers also discovered that the alcohol, shockingly, affected the breathing rate of these unborn children. The alcohol consumed by their mother caused a substantial drop in their breathing rate from the normal rate of 381 per hour to just 4 per hour.5

So, can you drink two glasses of wine when pregnant?

Two glasses of wine should be the absolute minimum; it might even be lower if you factor in body weight. So, should you drink alcohol whilst pregnant? Well, given what emerges from these studies regarding REM sleep, we know that it’s not optional for the fetus; it is an absolutely essential ingredient in the development of a healthy brain. We know from research on rat pups that deprivation of REM sleep disrupts their brain development, leading to a socially abnormal adult. So, based on what we have discussed and following on from the recommendations of the NHS and the CDC,

There’s no known safe limit for drinking alcohol during pregnancy

So, to answer the question we posed at the beginning of this article, can a pregnant woman drink two glasses of wine? The answer is an emphatic NO!

Here’s an excellent article from NHS Inform that should further convince you to put down that glass of wine.


  1. Armstrong, E.M., 1998. Diagnosing moral disorder: The discovery and evolution of fetal alcohol syndrome. Social Science & Medicine, 47(12), pp.2025-2042.
  2. Vogel, G. and Hagler, M., 1996. Effects of neonatally administered iprindole on adult behaviors of rats. Pharmacology Biochemistry and Behavior, 55(1), pp.157-161.
  3. Havlicek, V., Childiaeva, R. and Chernick, V., 1977. EEG frequency spectrum characteristics of sleep states in infants of alcoholic mothers. Neuropaediatrie, 8(04), pp.360-373.
  4. Coles CD 1993 Impact of prenatal alcohol exposure on the newborn and the child. Clin Obstet Gynecol 36: 255–266
  5. Mulder, E.J., Morssink, L.P., Van Der Schee, T. and Visser, G.H., 1998. Acute maternal alcohol consumption disrupts behavioral state organization in the near-term fetus. Pediatric Research, 44(5), pp.774-779.

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