One of these pairs of chromosomes, called the sex chromosomes X and Ydetermine the sex of the person. It might come in handy knowing how identical twins are formed. The cause of stillbirth is often unknown, but the rate of babies who are stillborn is higher in twins and multiple births. The twin at right in the image is shown in the coronal plane with the head pointing rightwards. If they occur early in fetal development, they will be present in a very large proportion of body cells. Also, the two umbilical cords have an increased chance of being tangled around the babies. Monozygotic twins split into two zygotes at some time very early in the pregnancy.

One twin, the 'donor' twin, is small and anemicthe other, the 'recipient' twin, is large and polycythemic.

Can Identical Twins Be Male and Female?

Dizygotic DZ or fraternal twins also referred to as "non-identical twins", "dissimilar twins", "biovular twins", and, informally in the case of females, "sororal twins" usually occur when two fertilized eggs are implanted in the uterus wall at the same time. Monozygotic twins can have markedly different epigenetic profiles. If they occur early in fetal development, they will be present in a very large proportion of body cells. Idioglossia has been found to be a rare occurrence and the attention of scientists has shifted away from this idea. There is no DNA-based zygosity test that can determine if twins are indeed mirror image. How Are Twins Formed? A woman gets pregnant again while already pregnant, resulting in multiple fetuses at differing developmental stages.