First trimester
3 weeks pregnant
This is the week a pregnancy genuinely starts. Fertilisation occurs, and over the following days the resulting cell divides repeatedly while travelling down the fallopian tube toward the uterus. You will have no idea any of it is happening.
Size of
pinhead
Length
0.1 mm
Weight
< 0.001 g
Your baby at 3 weeks
A single fertilised cell, the zygote, carries the full set of genetic material that determines everything from eye colour to blood type. It divides roughly every twelve hours as it travels, becoming a solid ball of cells called a morula and then a hollow sphere called a blastocyst.
By the end of the week the blastocyst reaches the uterus and begins implanting into the lining. This is the point at which pregnancy is medically considered established, and it is far from guaranteed — a substantial proportion of fertilised eggs do not implant successfully, usually without the person ever knowing a conception occurred.
The blastocyst has already differentiated. The inner cell mass becomes the embryo; the outer layer becomes the placenta.
How you may be feeling
You will almost certainly feel nothing. A pregnancy test will not be positive yet, because hCG is only produced once implantation begins and takes time to reach detectable levels.
Some people notice very light spotting around implantation, often lighter and briefer than a period. It is frequently mistaken for an early period, which is one reason dating by last menstrual period sometimes needs correcting at a scan.
Common symptoms around week 3
- Usually nothing at all
- Occasionally light implantation spotting toward the end of the week
- Mild cramping that is easy to mistake for a period arriving
Appointments and tests around now
Nothing is typically scheduled this week. If you take regular medication, this is a good point to check with your prescriber whether it needs reviewing in pregnancy — the earliest weeks are when it matters most.
What is normal, and what varies
Very early loss is extremely common and is almost always down to chromosomal factors that were present from the start. It is not caused by lifting something, by stress, or by anything you did or did not do that week.
Worth doing this week
Treat yourself as pregnant if you might be
This week is developmentally significant and predates any positive test. If conception is possible, it is sensible to behave as though it happened.
Review medications early
Some common medications are worth changing in pregnancy. Ask your prescriber rather than stopping anything on your own, which can carry its own risks.
Keep taking folic acid
Neural tube development is only days away now, and supplementation matters most before it happens.
Looking ahead
Next week implantation completes and hCG rises enough that a test may finally turn positive.
Nearby weeks
Not sure which week you are in? The week tracker works it out from the first day of your last period, and the due date calculator gives you an estimated due date.