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Within the ovary, follicle mature and grow in size and as they cross 10mm in size it has a tendency to rupture and release the ovum within it. This phenomenon is essential to make sure a quality ovum is released. This ovum is later on fertilized by a sperm following intercourse and results in pregnancy
From your description absence of follicle on 13th days implies it was ruptured and ovum was released. Susten
300mg is a progesterone
pill and when taken after fruitful intercourse enhances chances of pregnancy.
In simple terms, though it is not a clear picture of ovulation, it is likely that you have ovulated between 10th and 13th day and if there was intercourse during this time, susten will enhance your chances of pregnancy.
The remnants of ruptured follicle are resorbed to repeat the cycle again.
Hope this answers your query.
All the best