Holly Hansen, age 10, of Spokane, Wash., for her question:

CAN THERE BE IDENTICAL TRIPLETS?

Multiple births do not occur frequently in humans. It is usual for only one baby to be born at a time. The first quintuplets known to live for more than a few hours after birth were the five Dionne sisters who were born near Callander, Ontario, Canada, in 1934. Modern care and new medical knowledge have increased the survival rate of multiple birth children.

Multiple births can bring twins, triplets, quadruplets, quintuplets and even sextuplets. The first recorded set of sextuplets to survive early infancy were the six Rosenkowitz children who were delivered in Cape Town, South Africa, in 1974.

Twins and even triplets seem to run in families, leading scientists to believe there are hereditary elements at work with multiple births.

Sometimes twins will come from two different eggs supplied by the mother which merely happen to be fertilized and develop at the same time. If this type of birth occurs, the babies are called fraternal twins. Identical twins, on the other hand, are born from the cell mass originating from a single egg which has become divided.

If the cell mass separates before it begins to take shape, the identical twins will be almost exactly alike in every respect. The children are often called mirror twins. If the split comes after the cell mass has begun to take shape, the cell mass may already have begun to develop a left and right side. In this case, one of the identical twins may be left handed while the other is right handed.

Triplets are three children born to the same mother at the same time. There can be fraternal triplets, where each child has developed from a separate egg. There can also be identical triplets, where all three are born from one egg cell that has divided into three separate eggs.

Or the triplets may include a pair of identical twins plus a third fraternal child.

A set of triplets will be born only once out of 9,216 births. Twins, on the other hand, will be born about once in every 96 births. The higher number of babies, the higher the odds against the birth. Only one set of quadruplets is recorded for every 900,000 births, while the chance of having quintuplets is only one set in about 85 million births.

Fertility drugs have been used by some doctors since the 1960s to help women who have previously been infertile to become pregnant. The special drugs in some cases cause a woman's ovaries to release more than one egg. If fertility drugs are taken by a woman, her chance of having multiple births is greatly increased over the regular odds.