Your pregnancy period is calculated starting from the first day of your last menstrual period even though you are technically not pregnant at this point of time. This is because it is easier to know the first day of your period but it’s quite difficult to tell the time when fertilization and conception takes place.
Week 1 of your pregnancy is the week of your last menstrual period. Your uterine lining and the unfertilised egg from the last cycle is being discarded through the period. In a way your body is spring cleaning to pave the way for a new cycle.
This is the week of ovulation. A new egg will be released from the ovary into the fallopian tube where it will either wait for a sperm to mate with or meet with a waiting sperm. Sperm can hang out and stay fertile 3-5 days while an egg can survive upto 24 hours. When the twain shall meet – fertilisation shall occur and you will be technically pregnant!
Conception is fertilisation of Ovum (Female Egg) with Sperm to form a Zygote which is about the size of a poppy seed. This occurs by the end of the 2nd week.
This week you are officially pregnant though you might not know it for some time. Pregnancy symptoms will take another couple of weeks to show up, the earliest women have reported the early pregnancy symptoms is week 4! The fertilised egg gets busy almost immediately. There is so much to do and such little time! It splits multiple times, migrates to the uterus to set up home and “make the baby” from just a few cells. Within the week there is a flourishing cluster of about 100 cells collectively called a blastocyst which burrows itself into the uterine lining (endometrium). This is called implantation. This could happen towards the end of week 3 or in week 4.