Baby developing inside you
over the weeks and months
Week One: Your last menstrual period
This is the week of your last menstrual period. The first day of menstrual bleeding is considered the official start date of pregnancy and the 40 weeks of pregnancy are calculated by this date.
Not sure which week? Use the Weeks - Due Date Calculator and see how baby develops.
Pregnancy is broken down into weeks, and full term pregnancy is considered 40 weeks and an infant delivered before the 37th week completed is considered preterm (premature). Pregnancy is counted from the first day of a your last menstrual period (LMP) to birth, at about 40 weeks.
Pregnancy lasts 266 days from the moment the egg is fertilized. As actual fertilization is usually difficult to pinpoint precisely. When estimating how pregnant you are, day one of your pregnancy is taken from the first day of your last menstrual period (LMP) and not the day of fertilization.
If you have an average 28 day cycle, fertilization is counted as having taken place around day 14 and not day one of your pregnancy. This is because ovulation usually takes place about 14 days before the start of your period.
The pregnancy timescale is 266 days plus 14 days -- that is, 40 weeks. However this is only a guide as the average normal pregnancy can last anything from 38 to 42 weeks
The first, second, and third months are the first trimester of pregnancy The first trimester of pregnancy = Week 1 till end of week 13.
The first trimester of pregnancy = Week 1 till end of week 13.
Week by week: First trimester ultrasound scan pictures
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