The pregnancy week is determined by your ovulation day, which is when conception took place and the tiny fetus started growing from an egg cell, all the way to a baby.
The health care system counts by your last period’s first date, and assumes that you ovulated 14 days later – this is often not accurate, as many women do not ovulate on Cycle Day (CD) 14. They, later on, do ultrasounds to size the fetus and determine how far along you are and when you are due to give birth. This is why we do not count in this manner, and why your given date by Natural Cycles may not be the same as the one provided by the midwife.
When you realize that you are pregnant, you are already 4-5 weeks pregnant actually, depending on how early or late you take a pregnancy test. This is as a pregnancy test is only taken 10-15 days after ovulation (it takes the egg roughly 10 days to travel down the fallopian tube and attach to the wall of the uterus, at which point the pregnancy test can provide a positive result), and the pregnancy is calculated from 14 days prior to ovulation. This means that your baby is always 2 weeks ‘younger’ than the pregnancy weeks indicate.
An example:
You ovulated on CD 22 of your cycle, and take a positive pregnancy test on CD 36 – 14 days after your ovulation. You are 4+0 weeks – you have gone 4 full weeks of your pregnancy and is on the first day of your 5th week. You are 5 weeks along.
Here we see how the graph looks for a Plan user who has become pregnant. Ovulation is marked on CD 22, and the coming days have changed colour to blue – which is the pregnancy colour in the app.
Some women experience implantation bleeding, which occurs roughly 10-14 days after the ovulation, and can be as light as spotting or as heavy as your period. It is the fertilized embryo that's attached to the uterine wall, which may 'push away' some blood, which exits through the cervix.