There are two main types of twins, that I'm sure you're already familiar with - identical and fraternal.
The majority of twins are fraternal, otherwise known as non-identical or dizygotic (DZ) twins. Fraternal twins are the result of two different eggs being fertilised and resulting in two completely distinct and separate pregnancies in the womb at the same time.
On rare occasions, the two conceptions even occur one month apart!
Identical or monozygotic (MZ) twinning, on the other hand, occurs when a single conception splits into two around the time that the pregnancy's implanting in the womb (between the fourth and twelfth day of after fertilisation).
Monozygotic twins are identical in the sense that they have the same genetic makeup but they are not necessarily identical or even similar in all their behavioural and physical characteristics.
Parents are frequently mistaken about the twin type of their children. Studies show that parents have a 10 percent chance of being incorrect in determining twin type based on how similar their twins look and whether they could "tell the twins apart" as they were growing up.
Errors in the twin typing occur because dizygotic twins can look incredibly similar,since,after all, they are siblings with many genes in common.
Monozygotic (identical) twins sometimes look very different, especially at birth. Their weights tend to vary more than dizygotic twins' and this discrepancy in size can continue into adulthood.
Many traits, such as whether or not the twins are both right-handed, are not reliable indicators of twin type. (Handedness is different in 25 percent of monozygotic twins and in 25 percent of dizygotic twins.)
Sometimes parents are misled by their doctor or midwife who reports the twin type based on an incomplete or erroneous examination of the placenta.
Unless the twins are of different sexes (definitely fraternal) or inside a single bag of water, the amniotic sac (definitely identical), examining the babies or the placenta with the naked eye is not very helpful.
The number of placentas does not predict zygosity, since two-thirds of monozygotic twins have one placenta, and 42 percent of dizygotic twins have a single, fused placenta.
You can't guess the zygosity based on how thick or thin the membrane between the two amniotic sacs is, or by how easily it can be peeled apart.
Microscopic examination of the placenta, usually by a pathologist, is the only accurate way to use this intervening membrane to determine zygosity.
Unfortunately, even with a detailed examination, the placental membranes can reveal the zygosity of only 35 percent of same-sex twins.
Zygosity can also be determined by examining various simply-inherited traits like blood groups, HLA types, protein markers, DNA markers, and physical characteristics and measurements.
If the twins differ for any two traits, they are classified as dizygotic.
If all traits examined are the same, monozygosity is not proven, but the probability that the twins are monozygotic can be calculated.
For decades, twin testing was performed by testing dozens of major and minor blood "types" in both twins. Blood types are determined by the chemicals which cover our red blood cells.
If twins match for every blood type tested, then a calculation is done to determine if this occurred by chance alone.
But these tedious calculations depend on knowledge of the frequencies of each "type" in the twins' particular ethnic group. Even after many blood groups are examined, the probability of erroneously classifying twins as monozygotic can be five to ten percent.
Eventually, HLA tests and protein-typing were added to the battery of tests to make blood analysis more accurate.
The use of DNA polymorphisms has improved twin zygosity determination. DNA is the chemical of which our genes are made and DNA polymorphisms are sites in the DNA chain which are extremely variable from one person to the next.
Any two individuals have millions of differences in their DNA codes, unless they are monozygotic twins.
The newest type of DNA markers which have been developed for twin testing are called STAR (short tandem amplifiable repeat) markers.
If the twins match completely, they can be classified as "monozygotic" with greater than 99,98 percent probability.
The Western Province Blood Transfusion Services are able to perform the DNA-STAR test. Although the forensic laboratories in South Africa are doing it on very small samples of tissue or body fluid, the Blood Transfusion Services prefer to do the test using a blood sample.
The test's routinely used by the courts to establish paternity. Unfortunately, testing is fairly expensive. Contact your local Blood Transfusion laboratory if you're interested in determining your multiples zygosity.