<prompt> Commonest cause of first trimester abortion is:MonosomyTrisomyTriploidyAneuploidy  <response>: Ans. is d, i.e. AneuploidyRef. Williams 24e, p 351The MC cause of early miscarriages is aneuploidy i.e. chromosomal anomaly.Amongst the aneuploid abortions--autosomal trisomy is the most frequently identified chromosomal anomaly.Trisomies have been identified in abortuses for all except chromosomal number 1 and those with 16, 13, 18, 21 and 22 are most common.The single most specific chromosomal abnormality in abortions is Monosomy X 45X.As a have discussed Best answer for MC cause of 1st trimester abortion--Aneuploidy Trisomy Monosomy X Trisomy 16. 