On the Fetal Origin
Original Text
夫胎者，受形之始，故易乾知太始，以形言也；月者，成气之时，故传曰积日以为月，以气言也。今谈命或不以胎月为重，殊不思胎月是四柱之根苗，日时虽为紧，若不犯破胎月，或乘旺气禄马之处则为福尤多，或日时之吉而为胎月所犯，则吉亦归无用。是胎月最为枢要。
胎元之取，唯有一法，以当生前三百日为十月之气，乃是受胎之正，譬甲子日生便以甲子为受胎之日，盖五六计三百日，看其生日在何月中有，则闰在其中矣。且如戊子生人甲寅月乙丑日，须于半月前十月或十一月内，寻当生乙丑日，乃是三百日之正胎。
Interpretation
The fetus represents the beginning of the formation of the physical body. Therefore, in the "Book of Changes", it is said that "Qian" knows the great beginning, which refers to the physical form. The month is the time when qi is formed. Hence, it is stated in the commentary that days are accumulated to form a month, which refers to qi. Nowadays, when discussing one's fate, some people do not attach importance to the month of conception and the month of birth. They never think that the month of conception and the month of birth are the roots and seedlings of the four pillars of the birth chart. Although the day and the hour are also crucial, if they do not violate the month of conception and the month of birth, or if they are in a position of prosperous qi, good fortune and noble stars, then there will be even more blessings. However, if the auspiciousness of the day and the hour is violated by the month of conception and the month of birth, then the auspiciousness will become useless. Thus, the month of conception and the month of birth are of the utmost importance.
There is only one way to determine the time of conception. Take 300 days before birth as the qi of the ten - month period, which is the exact time of conception. For example, if someone is born on an A - Jia - Zi day, then the A - Jia - Zi day is regarded as the day of conception. Since five times sixty makes 300 days, check which month the birthday falls into, and the leap month is included in this calculation. For instance, if someone is born on a B - Yi - Chou day in the J - Jia - Yin month of the W - Wu - Zi year, one should look for the corresponding B - Yi - Chou day in the tenth or eleventh month about half a month earlier. This is the exact time of conception calculated based on 300 days.