The risk to your unborn is unheard of.
Caffeine consumption during pregnancy is discouraged. Generally speaking, caffeine functions as a stimulant and when ingested it affects various areas of the body. As a result, its effect on pregnancy can be adverse. When ingested, caffeine enters the body through the central nervous system and starts to affect the body by increasing blood pressure, heart rate, increasing urination and possibly dehydrating the body.
Keeping the body hydrated is important, especially during pregnancy, thus, caffeine's diuretic effects are undesirable during pregnancy. Caffeine consumption by pregnant women is associated increased fetal breathing activity and behavior, increased breech births, increased tremors and irregular heartbeat in new born babies.
In addition, caffeine increases the risk of pregnancy complications, such as gestational diabetes, pregnancy induced high blood pressure and leg cramps.
It was first found to be of potential harm to pregnant women by the US Food and Drug Administration in 1980. It was found that caffeine crosses the plancental and blood brain barrier and it is thought that the fetus may not have the enzymes necessary to detoxify itself of caffeine through a process known as demethylation. Some scientists have also tried to determine how caffeine interferes with fetal cell growth and development. Evidence shows that caffeine can produce chromosomal aberrations in human cells but this is the subject of ongoing debate.
Various epidemiological studies on the adverse effects of caffeine have shown that there is a weak to strong association - no studies have shown that there is no association. The risk of miscarriage in pregnant women who consume a cup or more of caffeine per day was demonstrated in a 1998 study - some cases caffeine can double the risk of miscarriage. Other studies have shown that caffeine may lead to decreased birth weight for the child and also an increased rate of spontaneous abortion.
Caffeine consumption, when combined with diseases such as diabetes, can compound the risks to the fetus. One study found that the more caffeine a pregnant woman with diabetes consumes during the first trimester of her pregnancy, the less likely her body is to release enzymes that signal maintenance of the pregnancy. This can be seen as an indication of the impending loss of the pregnancy and implies that over time, caffeine has negative effects on pregnancy.
A study in Denmark concluded that caffeine consumption can definitely lead to spontaneous abortion. This study confirmed the findings from previous studies and demonstrated that caffeine can indeed double the risk of spontaneous abortion.
While much of the caffeine and pregnancy studies focus on effects during pregnancy, there are some studies that indicate that caffeine can affect women even before they become pregnant. Some studies indicates that women who consume more than a cup of caffeine a day may have difficult conceiving - these women are twice as likely to take a year or longer to become pregnant.
Women have to completely avoid or limit their caffeine intake even after they have given birth. This is because breast milk can transfer caffeine from the mother to the baby. Caffeine ingestion through breast milk can make the baby irritable and affect the baby's sleep cycles.
Giannelli, M., Doyle, P., Roman E., Pelerin M., and Hermon C. "The effect of caffeine consumption and nausea on the risk of miscarriage." Paediatric and Perinatal Epidemiology 17.2003:316-323.
Khoury, J.C., Miodovnik M., Buncher, C.R., Kalkwafr, H., McElvy S., Khoury P.R., and Sibai, B. "Consequences of smoking and caffeine consumption during pregnancy in women with type 1 diabetes." The Journal of Maternal-Fetal and Neonatal Medicine 15.2004: 44-50.
Rasch, V. "Cigarettes, alchohol and caffeine consumption." Acta Obstetrica Gynecologica Scandinavia 82.2003:182-188.