It is generally thought that low to moderate caffeine consumption does not affect fertility or increase any risk of miscarriage or preterm labor.
In the case of pregnancy, moderate caffeine intake is considered to be less than 300 mg of caffeine a day. It is important to read the labels of the products you consume while pregnant, as their contents may vary greatly from one brand to another or one style to another. For example, coffee from a drip coffee maker may have anywhere from 90 to 290 mg of caffeine in each cup. And, Mountain Dew at 55 mg is not the only soda pop that is high in caffeine; most other soda pops have 45 mg or more.
You should avoid higher intake of caffeine during pregnancy. Caffeine passes easily from mother to fetus via the placenta. Some studies have shown that caffeine intake may increase the heart rate and movement patterns of your baby. Pregnant women who consume large quantities of caffeine (more than 5 cups of coffee a day) may have a greatly increased risk of miscarriage.
In addition, caffeine stimulates the bladder and generally increases the need to urinate. This can lead to dehydration in pregnant women.