First trimesterMost clinically apparent miscarriages (two-thirds to three-quarters in various studies) occur during the first trimester.About 30% to 40% of all fertilized eggs miscarry, often before a woman knows she is pregnant
Chromosomal abnormalities are found in more than half of embryos miscarried in the first 13 weeks.Chromosomal problems due to a parent's genes are, however, a possibility. This is more likely to have been the cause in the case of a woman suffering repeated miscarriages, or if one of the parents has a child or other relatives with birth defects.Genetic problems are more likely to occur with older parents; this may account for the higher rates observed in older women.
Progesterone deficiency may be another cause. Women diagnosed with low progesterone levels in the second half of their menstrual cycle (luteal phase) may be prescribed progesterone supplements, to be taken for the first trimester of pregnancy.No study has shown that general first-trimester progesterone supplements reduce the risk however, (when a mother might already be losing her baby), and even the identification of problems with the luteal phase as a contributing factor has been questioned.