In The Name Of God

Prenatal Diagnosis of Congenital Disorders Dr. Gita Hatamizadeh OBGYN- Infertility Fellowship Assisted professor of IAUTMB Jan. 2017

Prenatal diagnosis Prenatal diagnosis, a term once considered synonymous with invasive fetal testing and karyotype evaluation, now encompasses pedigree analysis, population screening, fetal genetic risk assessment, genetic counseling, and fetal diagnostic testing.

Although prenatal evaluation of the fetus for genetic disorders can have a huge impact on individual families, most screening and testing is done for events that occur in less than 1% of pregnancies.

An ideal perinatal genetic screening test should fulfill the following criteria: • Identify common or important fetal disorders • Be cost-effective and easy to perform • Have a high detection rate and a low false-positive rate • Be reliable and reproducible • Screen for disorders for which a diagnostic test exists • Be positive early enough in gestation to permit safe and legal options for pregnancy termination if desired.

Screening for Fetal Genetic Disorders Detecting or defining risk for disease in an asymptomatic low-risk population is the goal of screening. As opposed to diagnostic testing, intended to identify or confirm an affected individual, screening is intended to identify populations who have an increased risk for a specific disorder, and for whom diagnostic testing may be warranted.

Sensitivity and specificity are two key concepts in screening test performance. Sensitivity is the percentage of affected pregnancies that are screen positive. Specificity is the percentage of individuals with unaffected pregnancies who screen negative. The reciprocal of specificity is the false-positive rate.

Preimplantation genetic diagnosis and screening (PGS)

Preimplantation genetic diagnosis (PGD) has been developed for patients at high risk of transmitting a genetic abnormality to their children, which includes all monogenic defects (autosomal recessive, autosomal dominant, and X-linked disorders).

Preimplantation genetic screening (PGS) Applies the same technology in couples having no known chromosomal or genetic abnormality in efforts to identify and exclude aneuploid embryos. The beneficial effect of PGS was expected to be greatest in:

1- women of advanced maternal age, since aneuploidies occur more frequently in woman ages> 35 years of age and also in these women pregnancy chances decline sharply both in normal conception and after IVF . 2- women with a history of recurrent miscarriage, 3- women with a history of RIF (several failed IVF cycles), 4- women with a partner with low sperm quality (severe male factor).

It has long been known that women over the age of 35 years are at increased risk of having a fetus with a chromosome abnormality, and this is probably due to an age-related increase in oocyte aneuploidy. Chromosome abnormalities are the primary cause of embryo wastage in patients aged >35 years and up to 80% of embryos in women over 40 years may be aneuploid.

Therefore, screening oocytes or embryos for aneuploidy so that only chromosomally normal embryos are transferred should increase implantation rates, reduce spontaneous abortion, and increase live-birth rates. But we saw in women of advanced maternal age PGS significantly lowered the live birth rate.

Recently, it was concluded that PGS did not increase but instead significantly reduced the rates of ongoing pregnancies and live births after IVF in aged women. There were, however, problems with the study design and implementation that may have led to the damage of biopsied embryos leading to lower viability.

A relationship between maternal age and aneuploidy for chromosome 16 was also identified. By considering only these data, rates of aneuploidy in women aged 40 years would be expected to exceed 40%. There remains a possibility that the rate of embryonic aneuploidy may be even higher in these women when further assessment of additional chromosomes is included.

PGD Technique Each embryo is biopsied on day 3, in which one to two blastomeres are removed from an 8- to 10-cells embryo. Analyses of two blastomeres may be preferable to reduce misdiagnoses. Early work suggested that removing single blastomeres from eight-cells embryos did not affect their viability or ability to progress to the blastocyst stage.

Following blastomere biopsy, the cells are fixed on a slide and analyzed by FISH, with a multiple probe technique in a time frame compatible with clinical IVF (embryo transfer on day 4 or 5). Up to five chromosomes can be detected by FISH at the single-cell level.

Cells are analyzed using fluorescent probes for chromosomes X, Y, 13, 15, 16, 17, 18, 21, and 22. Only normal embryos are transferred on day 4 or 5. For women aged ~35 years, PGS significantly reduced pregnancy losses and increased the number of viable pregnancies. We are able to achieve acceptable ongoing pregnancy rates with PGS until the age of 42 years (28.8%), but women >42 years has a poorer prognosis.

The identification of high-quality embryos in all women undergoing IVF, including poor responders, remains a high priority. In the future, accurate noninvasive methods for assessing oocyte and embryo quality may also become available, such as gene expression profiling of cumulus cells surrounding the oocyte, and metabolomics and proteomics.

Preimplantation Genetic Diagnosis (Aneuploidy Screening) There is significant evidence that implantation failure in women of advanced maternal age is closely linked to embryonic aneuploidy. Utilizing blastomere biopsy we saw even in embryos judged to be of good quality, aneuploidy rates were 4.0%, 9.4%, and 37.2%, in women aged 20-34, 35-39, and 40-47 years, respectively.

Methods have been developed for the detection of aneuploidy in older women undergoing IVF-ET. These methods were developed to improve the implantation efficiency, reduce the spontaneous abortion rate, as well as decrease the incidence of chromosomal abnormalities at term. The first method is FISH technique.

Recent work has included the use of comparative genomic hybridization (CGH) on single blastomeres for the purpose of aneuploidy screening, which allows for the simultaneous assessment of every chromosome in single interphase cell. Chief limitations of single-cell CGH are that it is complex and requires 3-4 days to complete, thereby requiring initial freezing of biopsied embryos and later thawing prior to transfer.

The use of first polar body analysis for aneuploidy detection has also been proposed as an alternative to blastomere analysis, which is hampered by the inability to diagnose paternally derived chromosome abnormalities as well as those resulting from postfertilization events.

However, recent RCTs found that there was no evidence of a beneficial effect of PGS on the live birth rate after IVF. It was concluded that application of PGS should be evaluated carefully before their introduction into clinical practice.