To begin with, it is worth considering that the period of conception and the period of pregnancy differ. The gestational age is formally counted from the first day of the last menstrual period. Normally, it is 10 lunar (28 days each) months of pregnancy or 40 weeks. Pregnancy is divided into three trimesters of 13 weeks, and at the end of the 3rd, at 40 weeks, birth is expected... theoretically, because nature does not always agree with mathematics. A baby is considered full-term from 38 weeks of pregnancy and has the right to sit in the mother’s belly until 42 weeks. Further, the pregnancy is considered post-term. From 22 weeks, doctors try to save the life of the child if he was born prematurely.
Conception normally occurs from 8 to 18 days of the menstrual cycle - an egg is released from the ovary and the sperm has a chance to meet it. But options are also possible. The cycle sometimes shifts due to abortion or miscarriage, hormonal drugs, illness, stress, climate change. Occasionally, both ovaries release an egg, and twin children can be conceived within a few days of each other. And with a rare pathology - duplication of the uterus - the difference in timing can be as much as a month. In the BBC story of Arifa Sultana from Bangladesh, the woman gave birth to one child in February 2019, and two more in March.
Sometimes a woman makes the mistake of mistaking implantation bleeding for her last menstrual period. Or cannot remember the dates of the last menstruation due to a lost or irregular cycle. And we should not forget about conception during the period of lactational amenorrhea. In these cases, standard counting does not work.