A human pregnancy lasts approximately 40 weeks (280 days) from the last menstrual period, or 38 weeks (266 days) from the date of fertilization. The 38 week gestation period is approximately 10 lunar months, i.e 27.3 days/lunar-month x 10 = 273 days. In most parts of the world, according to the calendar, 40 weeks from the last menstrual period is equivalent to a little more than nine months and six days.