GUIDE
Your baby is the size of a poppy seed — a tiny cluster of cells traveling toward the uterus for implantation.
Fertilization happened last week, and now the single-celled zygote is dividing rapidly as it makes its way down the fallopian tube. By the end of this week, it will be a ball of about 100 cells called a blastocyst, burrowing into the uterine lining. You probably have no idea any of this is happening — most people feel completely normal at week 3.
Be prepared from day one
Your baby is about the size of a poppy seed — but calling it a "baby" at this stage is generous. Right now, it is a rapidly dividing cluster of cells on an extraordinary journey.
After fertilization in the fallopian tube, the single-celled zygote began dividing immediately. By day 2, it was 2 cells. By day 3, it was a solid ball of 12-16 cells called a morula. By day 5-6, it has become a blastocyst — a hollow sphere of about 70-100 cells with a fluid-filled cavity. The blastocyst has two distinct parts: the inner cell mass, which will develop into the embryo, and the trophoblast, the outer ring of cells that will become the placenta.
Around days 6-10 after fertilization, the blastocyst reaches the uterus and begins implantation — burrowing into the thick, blood-rich uterine lining (endometrium). The trophoblast cells attach to the uterine wall and begin to invade the lining, establishing the earliest connections with your blood supply. This is when hCG (human chorionic gonadotropin) production begins — the hormone that pregnancy tests detect.
For a broader view of what led to this moment, see our week 2 guide.
Remarkably, you probably feel nothing different. Implantation is happening at the microscopic level, and hCG production is just beginning — far too low to cause the symptoms most people associate with early pregnancy.
Your corpus luteum (the structure left behind in the ovary after ovulation) is still producing progesterone, which keeps the uterine lining thick and receptive. Progesterone also keeps your basal body temperature slightly elevated. If you have been charting your temperature, you may notice it stays high rather than dropping back down as it would before a period.
Some people notice very light spotting around this time — often called implantation bleeding. It is caused by the blastocyst disrupting small blood vessels as it burrows into the uterine lining. This spotting is typically pink or light brown and lasts only a day or two. It is much lighter than a period. However, many people experience no implantation bleeding at all, so its absence does not mean anything.
Every pregnancy is different. You may experience all, some, or none of these symptoms.
The most important thing you can do this week is continue the healthy habits you have already established — and resist the urge to take a pregnancy test too early.
The neural tube begins forming very soon — within the next week or two. Adequate folic acid right now is critical. If you are not already taking a prenatal vitamin with at least 400 mcg of folic acid, start today.
Even though you may not know you are pregnant yet, the embryo is exquisitely sensitive to toxins during the earliest stages of development. If you are trying to conceive, treat this window as if you are already pregnant.
A balanced diet rich in folate, iron, calcium, and omega-3 fatty acids supports the earliest stages of embryonic development. Leafy greens, lean protein, whole grains, and healthy fats are your best allies.
It is tempting to test early, but a pregnancy test taken this week will almost certainly be negative even if you are pregnant. hCG needs time to build up after implantation. Waiting until at least the day of your expected period will save you from misleading results and unnecessary anxiety.
When in doubt, call your provider. No question is too small during pregnancy.
Next week, implantation will be complete, hCG levels will begin rising rapidly, and you may finally be able to get a positive pregnancy test. This is when things start to feel real.
Next up: Week 4. Wondering when you can test? Check out our when to take a pregnancy test guide. You might also be watching for early signs of pregnancy. And if you are curious about the hormone that pregnancy tests detect, our guide on hCG levels in early pregnancy explains what to expect.
This guide is for informational purposes only and is not a substitute for professional medical advice. Always consult your healthcare provider with any questions about your pregnancy.
