Twins have a high incidence of spontaneous early pregnancy loss, one study suggesting 12% of human conceptions start off as twins . Studies of ultrasound or abortal pathology indicate that twins are found twice as commonly in the first trimester as at birth. First trimester resorption of one previously ultrasonically viable twin known as the 'vanishing twin' phenomenon is estimated to occur in over 20% of twins . Spontaneous first trimester loss of one or more fetuses in high-order multiple pregnancies is common, with nearly 40% of pregnancies in which three sacs are seen in early pregnancy delivering twins. When one twin dies in utero in the mid-trimester, a fetus papyraceus, its squashed paper-like remains may be found among the placenta after delivery.
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The first trimester is very important for the mother and the baby. For most women it is common to find out about their pregnancy after they have missed their menstrual cycle. Since, not all women note their menstrual cycle and dates of intercourse, it may cause slight confusion about the exact date of conception. That is why most women find out that they are pregnant only after one month of pregnancy.