Just as the external physical body shows signs of aging, such as wrinkles and gray hair, the internal organs and cells also age. Thus, maternal age is one of the most determinant factors in regards to pregnancy success rates and live births. At the age of 40, it is estimated that as much as 30% of embryos are aneuploidy (a cell with an abnormal number of chromosomes), while at the age of 45, this increases to almost 100%. Thus the incidence of embryos with chromosomal abnormalities increase as maternal age increases. Most aneuploidies arise from errors in meiosis, typically due to nondisjunction (inability of a chromosome to properly separate) and account for approximately 50% of first trimester losses, 30% of second-trimester losses, and 3 % of stillborn births. Aneuploidy also accounts for unsuccessful IVF cycles when embryos are not screened.
Amongst the lethal aneuploidy category, 35% cause spontaneous abortions (such as 45X and trisomy 16, 21 and 22); while about 4% cause stillborn births (such as trisomy 13, 18 and 21). Of those aneuploidies that are somewhat compatible with life, trisomy 21, also known as Down syndrome, is the most common autosomal trisomy. Individuals with Down’s syndrome typically show signs of cognitive impairment, heart defects, and muscle weakness. The only other two autosomal trisomies that are detected in appreciable numbers are 13 and 18, however, affected individuals rarely survive the first few months of life.
On the other hand, Klinefelter’s syndrome (male 47, XXY) is an example of a sex chromosome trisomy. Affected individuals commonly show reduced sexual development and fertility, but they often have somewhat normal life spans. Monosomies are the opposite of trisomy, in that individuals affected have one chromosome less, 45 instead of 46. One uncommon monosomy is Turner syndrome (female 45, X0). Affected females have an array of symptoms, as there are a few variations of Turner’s syndrome (fully affected vs mosaicism), but typically include infertility, impaired sexual development, short stature, and heart defects.
Current Available Intervention
Couples who have conceived an embryo with an abnormal karyotype in the past, regardless of the pregnancy outcome, may be offered IVF with preimplantation genetic diagnosis (PGD) or comprehensive chromosomal screening (CCS). PGD encompasses both screening and diagnostic measures, which aims to analyze, select and transfer only embryos that have the appropriate number of chromosomes. PGD has been found to reduce the rate of miscarriage once pregnancy is achieved, but its ability to provide a better outcome for live birth compared to natural conception over time is controversial.
Chromosomal Abnormalities: Aneuploidies | Learn Science at Scitable. (2017). Nature.com. Retrieved 11 August 2017, from https://www.nature.com/scitable/topicpage/chromosomal-abnormalities-aneuploidies-290
Bashiri, A., Harlev, A., & Agarwal, A. (2016). Recurrent Pregnancy Loss. Cham: Springer International Publishing.