A woman is in her third trimester, or 12 weeks of gestation, when the fetus has taken its first human shape, the placenta has been created, and the baby has stabilized in all aspects of development. Under normal circumstances, the fetus in the third trimester can reach a size of between 65 and 90 mm and weigh around 60 to 70 grams. From the beginning of the third trimester, the baby will develop very fast in all aspects and will move inside the belly, but because the baby is still relatively small, the pregnant woman herself will not be able to feel the fetal movement. The baby’s skeletal development accelerates at this time, so pregnant women need to take calcium supplements. First of all, in the third trimester, the fetus can reach a length of about 90 mm and a weight of about 60 grams, so from the outside, the pregnant woman’s stomach does not change much. The baby’s toes are fully developed, nails and teeth start to form, and external genitalia also start to develop, this stage is the most crucial for the fetus’ development. All the baby’s organs have started working and the liver is able to secrete bile on its own. A woman should undergo her first prenatal checkup in the third trimester, establish a maternity health card, and then have regular prenatal checkups as required by her doctor. If the weight and length of the fetus in the third trimester do not correspond to the gestational week, it may be due to irregular menstruation and delayed ovulation, or if the weight and length of the fetus in the third trimester do not correspond to the gestational week, it may be due to a miscalculation of the gestational week and the correct gestational week needs to be calculated by examination; or if there is an abnormality in the fetus, the cause needs to be identified by a doctor.