Embryonic abnormalities occur very early in pregnancy, sometimes even before the mother knows she has become pregnant. Embryonic abnormalities can result in devastating congenital birth defects. Some birth defects can be prevented before pregnancy occurs by preconception care or pre-implantation genetic diagnosis (PGD).
Congenital birth defects have links to abnormal embryo development, caused by either genetic or environmental factors. Genetic defects can arise due to errors in gene re-arrangement during fertilization or due to spontaneous genetic mutations. For example, the presence of an extra chromosome 21 (trisomy 21) creates an abnormal number of chromosomes in the early embryo leading to Down's Syndrome. Other kinds of trisomies don't prove compatible with embryonic development, resulting in early pregnancy losses. Embryonic abnormalities can also result from environmental factors experienced by the embryo in the uterus such as malnutrition, infectious diseases or toxic exposures. Some embryonic abnormalities arise from a combination of genetic and environmental factors, says the National Institute of Neurological Disorders and Stroke (NINDS)
Severe embryonic abnormalities can occur during the first three weeks of embryonic development. During week one, the embryo can suffer abnormalities due to errors arising from faulty production of the egg and sperm or errors in fertilization when egg and sperm come together. Embryonic abnormalities that arise in week two have to do with problems during implantation of the blastocyst stage embryo in the uterus. NINDS says that starting in week three, the basic systems of the future fetus are laid down, so early embryonic abnormalities during week three to week twelve can cause severe congenital abnormalities.
According to National Institute of Neurological Disorders and Stroke, anencephaly is a severe congenital defect in which most of the brain does not develop although some babies are born with minimal brain stem function. These infants do not gain consciousness and typically die soon after birth. This devastating birth defect takes place because of an embryonic abnormality occurring between day 23 and 26 of embryonic development. During these four days, the early embryo forms and closes the neural tube which is the precursor structure for the entire nervous system, including the brain.
Traditional methods for identification of embryonic abnormalities through ultrasound imaging or genetic testing of placental cells are typically used when the pregnancy is well on its way, requiring couples to make difficult choices about termination of the pregnancy if test detect abnormalities.
Some early embryonic defects can be identified using a newer technique called preimplantation genetic diagnosis (PGD) which tests embryos created from in vitro fertilization (IVF) procedures. Embryos in the lab are tested for known genetic abnormalities and affected embryos can be excluded from the pool of embryos used to create a pregnancy. The Genesis Genetics Institute says that some IVF patients find this early intervention to prevent pregnancy, rather than terminate an existing pregnancy, more ethically acceptable.
Abnormal embryonic development can be prevented in some cases. For instance, according to National Institute of Health, taking folic acid as a dietary supplement before conception and during early pregnancy proves very effective in preventing neural tube defects.
The March of Dimes organization recommends that women who plan to become pregnant and should get vaccinated against the disease Rubella, also called German measles, to prevent embryonic abnormalities that result in birth defects of the eyes, ears, heart and mental retardation.