Mars, the planet of action and energy, goes into Cancer for the first time in two years on July 14, according to Miller's ephemersis, which means the baby would be a bundle of energy and confidence. Mars in one's sign denotes leadership.
"Mars hasn't been in Cancer in two years. It will stay [in Cancer through Aug. 27]," Miller said. "This child will have a lot of energy and leadership qualities."
With Saturn, the planet of self-discipline and longevity, in Scorpio in mid-July, a water sign like Cancer, a prudent newborn is in the cards.
"Saturn makes you slow down and think," Miller said, "so they won't have a reckless little baby here. I don't think it's a party baby. I think this baby is more like his or her parents."
Another factor to consider is Cancer's ruling planet: the moon. The female planet, which governs the sign, tends to make Cancers more emotional, maternal and nurturing people. It also speaks to the influence of the mother, in this case, Capricorn Kate.
Middleton's and William's signs -- Capricorn and Cancer -- are exact opposites, but complement each other perfectly, astrologically speaking, according to Miller.
"This baby is going to get along so well because the baby's mother is a Capricorn and he's Cancer," Miller said. "Father and mother make a beautiful pairing."
Once the baby arrives, the astrological puzzle can be more thoroughly pieced together (rising sign, moons and more) and even more can be gleaned about the child's personality, how he or she relates to parents and others.
Check back for the royal baby's full astrology chart after the birth.
If this baby is born after July 22, he or she will be a Leo, a sign that draws strength and authority from its ruling sign, the sun. Traditionally more male in temperament than Cancers, Leos are strong and courageous like the lion icon that represents them, and also quite regal like the rulers of the animal kingdom, Miller said. Leos are also generous, sophisticated and wield power with kindness.