The fetuses may develop from the same egg or different eggs. In cases where they develop from a single egg, the fertilized egg divides into two or more parts. These fetuses are said to be monozygotic or identical. In other cases, different sperms from the father fertilize different eggs released from the ovaries in the same cycle. These fetuses are said to be non identical or dizygotic or fraternal.
In last two decades, the number of twin births in the US has increased. This could be due to the advancing maternal age at conception and the fact that more couples are undergoing infertility treatment.
Multiple pregnancy is usually detected through ultrasound. Common symptoms associated with multiple pregnancy include excessive weight gain, increased appetite, a larger-than-expected abdomen size, excessive fatigue, severe morning sickness and fetal movement felt at different parts of the abdomen at the same time.
Multiple pregnancy can pose some common risks to the mother as well as the babies. It has been associated with pre-term labor, anemia, low birth weight, eclampsia, gestational hypertension, gestational diabetes, congenital abnormalities, miscarriage, twin-to-twin syndrome etc. Hence it is quite important to be aware of complications during multiple pregnancy and be prepared to take best possible measures to care for the mother and the developing babies.
Types of Multiple Pregnancies
Based on the origin of the multiple embryos, multiple pregnancy has been classified as identical or fraternal.
• Identical or Monozygotic Twins: Identical twins result when a woman develops two embryos from a single fertilized egg. The fertilized egg splits into two embryos a few days after fertilization. Identical twins share the same genes and are of the same sex. The occurrence of identical twins is unpredictable, and not influenced by external or internal factors like heredity, age, race, previous pregnancies, fertility treatment etc.
• Fraternal or Non-Identical or Dizygotic Twins: Fraternal twins result when a woman develops two embryos from two different fertilized eggs. Unlike identical twins, fraternal twins do not share the same genes. The occurrence of fraternal twins may be influenced by factors like race, age, previous pregnancies, fertility treatment etc. The sex of the babies may be the same or different.
• Higher Order Pregnancies: This type of multiple pregnancy results in triplets or more babies. It occurs when a woman develops more than two embryos that arise from one, two or more fertilized eggs. The developing babies can be all identical, non-identical, or mixture of identical and non identical.