Pregnancy › Week by Week › Week 3
This is the week it truly begins. Conception has just happened, and your baby exists as a tiny cluster of cells traveling down the fallopian tube toward the uterus. It is far too small to see and far too early to test, but the earliest chapter of development is under way.
Size
Smaller than a poppy seed
a cluster of cells
Length
About 0.006 in
microscopic
Weight
Too small to weigh
just cells
Your baby at 3 weeks
A fertilized egg has divided into a ball of cells called a blastocyst, traveling toward the uterus to implant.
When sperm met egg at the end of last week, a single cell formed, carrying the full set of genetic instructions, including whether your baby is a boy or a girl. That one cell has been dividing steadily ever since, and by week three it is a hollow ball of cells called a blastocyst.
The journey. The blastocyst spends this week traveling down the fallopian tube toward the uterus. Along the way it keeps dividing, and its cells begin to sort into two groups: an inner cluster that will become your baby, and an outer layer that will become the placenta and supporting structures.
Not implanted yet. For most of week three the blastocyst is still on the move, not yet attached to the uterine wall. Implantation usually happens at the very end of this week or the start of week four. Until it implants and starts producing the pregnancy hormone, there is nothing a test can pick up.
How big is your baby at 3 weeks?
Microscopic, smaller than a poppy seed, a ball of cells about the width of a few hairs.
At three weeks your baby is a cluster of cells too small to see without a microscope, measuring around a tenth of a millimeter, roughly 0.006 inches. It is even smaller than the poppy seed comparison you will see at week four, because it is still just a ball of dividing cells rather than an embryo with a body. There is nothing to feel and nothing to detect, but the growth happening at this scale is remarkable.
Your body at 3 weeks
You likely feel completely normal, since it is far too early for pregnancy symptoms.
Most people notice nothing different in week three. Conception has just happened, and the pregnancy hormone that eventually causes symptoms is not being produced yet, so you will not feel pregnant. Your period is not even late, since it is not due for about another week, which is why the two week wait can feel so long when you are trying to conceive.
Behind the scenes, the corpus luteum, the structure left behind after ovulation, is producing progesterone to keep the uterine lining thick and welcoming for implantation. A small number of people notice light spotting when the blastocyst implants, often called implantation bleeding, but this usually happens right at the end of week three or into week four, and many people have none at all. Feeling nothing this week is completely normal.
What is normal, and when to call your provider
Feeling normal is expected this week. If you already suspect you are pregnant, contact your doctor or midwife the same day for any of these:
- Sharp or severe one sided pain in the lower abdomen, which can be an early sign of an ectopic pregnancy
- Bleeding that is heavier than light spotting, especially with pain
- Dizziness or fainting alongside abdominal pain
- A fever over 100.4°F (38°C) with pelvic pain
- Any symptom that worries you, since early reassurance is always worth asking for
Your week 3 checklist
- Keep taking your prenatal vitamin with folic acid every day, it matters most right now
- Continue avoiding alcohol, smoking, and anything your provider has flagged
- Hold off on testing for now, since it is too early for an accurate result
- Stay steady with balanced meals, hydration, and rest while you wait
- Note the date your period is due so you know when a test will be reliable
Tests and scans around 3 weeks
It is too early to test. The pregnancy hormone is not detectable until after implantation, closer to a missed period.
A home pregnancy test measures hCG, the hormone the developing placenta produces only after the blastocyst implants. In week three implantation has usually not happened yet, so even the most sensitive test will read negative. Testing now often leads to a false negative and unnecessary worry. Waiting until around the day your period is due, which is roughly week four, gives a far more reliable answer, and a positive test lets you estimate your due date from your last period. There are no scans this early either, since there is nothing an ultrasound could yet see.
A few things worth having on hand now
A note for your partner
This is the start of the two week wait, and it can be a strangely quiet, anxious stretch for both of you. There is nothing to do but wait, which is harder than it sounds. Partners can help by not fixating on early testing, keeping the mood calm, and being a steady presence. Whatever the result, going through the wait together makes it lighter.
You are not alone this week
Waiting to find out, with no symptoms to read and no test yet reliable, is one of the hardest parts of early pregnancy. If you are watching for every twinge, you are far from alone. Be gentle with yourself this week, keep up your healthy habits, and know that whatever happens, you are doing everything right.
3 weeks pregnant, common questions
Can I get a positive test at 3 weeks?
Almost never. The pregnancy hormone hCG is only produced after implantation, which usually happens at the very end of week three or in week four. Testing now typically gives a false negative. Waiting until around your missed period is far more reliable.
Is my baby a poppy seed at 3 weeks?
Not quite yet. The poppy seed comparison belongs to week four. At three weeks your baby is an even smaller cluster of cells, microscopic and still traveling toward the uterus to implant.
What is implantation bleeding?
Some people notice light spotting when the blastocyst attaches to the uterine lining, usually at the end of week three or into week four. It is lighter than a period and short lived. Many people have none at all, which is equally normal.
Is the baby’s sex decided already?
Yes. The genetic sex was set at conception, when the sperm and egg combined. It will not be visible on a scan for many weeks, but the instructions are already in place from the very first cell.
Sources
- American College of Obstetricians and Gynecologists (ACOG), first trimester care and early pregnancy guidance.
- Mayo Clinic, Fetal development: the 1st trimester.
- MedlinePlus, National Library of Medicine (NIH), Fetal development and prenatal care.
Medical disclaimer: This page is for information and education only and is not a substitute for professional medical advice. Every pregnancy is different. Always follow the guidance of your doctor, midwife, or qualified provider, and contact them with any concerns.