In astrology, a Natal chart is the map of sky at the date and time of birth. By using this Natal chart there is a way to plot the Conception chart. Conception chart is the map of sky at the date and time when that native was conceived. There are laws to calculate the pregnancy period using the phase of moon and the ascendant and the Moon's position in the Natal Chart.
You can see the laws by visiting the site http://www.vedicstudy.com/conception.aspx
A chart allegedly based on the Chinese conception chart, and having appeared in Mothering magazine, claims to predict the gender of the child based partly on the age of the mother at conception. Doctors generally consider the chart's premise to be pseudoscientific, and its method to be pseudomathematical.