Our Baby Journey3 weeks pregnant
3 weeks pregnant is in the first month of pregnancy. You are in week 3 of 40, in the first trimester. Here is what is happening with your baby and your body this week.
Your baby at 3 weeks
The fertilized egg divides rapidly into a ball of cells (a blastocyst) as it travels down the fallopian tube toward the uterus.
This is the week conception usually happens. When a sperm fertilizes the egg in the fallopian tube, the two combine their genetic material into a single cell called a zygote. At this moment your baby's genetic sex and many inherited traits, such as eye color and blood type, are already determined.
Within about a day, the zygote begins dividing: two cells, then four, then eight, while being gently moved toward the uterus. After a few days it becomes a solid ball of cells, then a hollow blastocyst. The blastocyst has an inner group of cells that will become the embryo and an outer layer that will become the placenta.
Around six to ten days after ovulation, the blastocyst begins burrowing into the thickened lining of your uterus. This is implantation, and it continues into week 4.
Your body at 3 weeks
You won't feel pregnant yet. Some people notice light spotting when the embryo implants.
Most people feel nothing different this week. Once implantation starts, the outer cells of the blastocyst begin producing human chorionic gonadotropin (hCG), the hormone pregnancy tests detect. Levels are still far too low to measure at home, though. A small amount of light pink or brown spotting, called implantation bleeding, happens for some people, and mild cramping is possible. Progesterone from the corpus luteum may already be causing slight bloating or breast tenderness. Because these signs closely resemble premenstrual symptoms, there is no reliable way to tell yet.
Your belly at week 3
In month 1 (weeks 1–4) there is no visible change to your belly, and your uterus has barely changed size.
Belly size and shape vary a lot depending on your build, whether this is your first pregnancy, your baby's position and the amount of amniotic fluid. At checkups your doctor or midwife may measure your fundal height to track your baby's growth, and may arrange an ultrasound if the measurement is quite different from what they expect.
Symptoms at 3 weeks pregnant
- Usually no noticeable symptoms
- Possible light spotting (implantation bleeding)
- Mild cramping
- Slight bloating
- Breast tenderness
Tips for week 3
- Avoid alcohol from now on — no amount is known to be safe in pregnancy.
- Limit caffeine to about 200 mg a day (roughly one or two cups of coffee).
- Keep taking your prenatal vitamin.
Common symptoms in the first trimester
The first trimester runs from week 1 to week 13. It is when your baby's major organs, spine and heart form, and when many people feel the most tired and nauseous. Not everyone has every symptom, and it is normal for symptoms to come and go. Common ones include:
- Nausea and vomiting (morning sickness), which can happen at any time of day
- Tiredness and needing more sleep
- Tender, swollen breasts
- Needing to urinate more often
- Food aversions or cravings and a metallic taste
- Mood swings
- Light spotting around implantation
Read the full first trimester guide.
When to call your doctor or midwife
Contact your healthcare provider or maternity unit straight away, or get emergency care, if you have:
- Vaginal bleeding
- Severe or persistent abdominal pain or cramping
- A severe headache, blurred vision, or sudden swelling of your face, hands or feet
- A fever of 38°C (100.4°F) or higher
- Fluid leaking from your vagina
- Severe vomiting where you cannot keep fluids down
- Your baby moving less than usual or a change in their pattern of movements (later in pregnancy)
- Regular contractions before 37 weeks
- Pain or burning when you urinate
- Feeling very low, anxious, or having thoughts of harming yourself
Have a question about week 3?
Ask our AI pregnancy assistant about symptoms, nutrition, scans or anything else on your mind this week.
Frequently asked questions
How big is my baby at 3 weeks pregnant?
At 3 weeks your baby is microscopic: a tiny ball of cells called a blastocyst, on its way to implant in the uterus.
How big is my belly at week 3 of pregnancy?
In month 1 (weeks 1–4) there is no visible change to your belly, and your uterus has barely changed size. Bump size, shape and when you start to show vary a lot from person to person.
Can I take a pregnancy test at 3 weeks?
It is usually too early. hCG only starts being produced once implantation begins, and it takes several more days to rise to a level home tests can detect. Testing now may give a negative result even if you are pregnant. For the most reliable answer, wait until the day your period is due or later.
How can I tell implantation bleeding from a period?
Implantation bleeding is typically lighter, pink or brown rather than red, and lasts a few hours to a couple of days. It usually does not include clots or build up the way a period does. It also tends to appear a little before your period would be due. If you are unsure, take a pregnancy test after your missed period.
Can you have pregnancy symptoms at 3 weeks?
A few people notice mild cramping, spotting, or tender breasts, but most feel nothing at this stage. These signs are also very similar to premenstrual symptoms, caused by the same hormone, progesterone, so they cannot reliably tell you whether you are pregnant. A test after your missed period is the best way to know.
What trimester is 3 weeks pregnant?
Week 3 is in the first trimester, which covers weeks 1 to 13.
How many months is 3 weeks pregnant?
3 weeks pregnant is in the first month of pregnancy. The first month covers weeks 1 to 4. Pregnancy is about 40 weeks, or roughly 9 months, counted from the first day of your last period.
This page is general educational information and is not a substitute for advice from your doctor or midwife. Every pregnancy is different, and baby sizes are approximate averages. Last updated 2026-11-07. See our medical disclaimer.