Congratulations! At 3 weeks pregnant, you’ve officially conceived and are in month 1 of your pregnancy — though it will be a few weeks until you can confirm the news with a pregnancy test.

This week, the tiny cluster of cells (soon to become your baby!) are rapidly growing. Meanwhile, surging hormone levels may trigger a heightened sense of smell, one of the earliest signs of pregnancy.

Your Baby at Week 3

At a Glance

Baby on board!
Baby on board!
We have an embryo! Your soon-to-be fetus is still a cluster of cells that are growing and multiplying. It’s about the size of a pinhead.
The journey begins
The journey begins
It takes about four days for your fertilized egg — now dubbed a blastocyst — to reach your uterus and another two to three days to implant.
Boy or girl?
Boy or girl?
You probably won’t know if you’re having a baby boy or girl for about 14 more weeks, but sex is determined at the moment of fertilization.

3 weeks pregnant is how many months?

If you’re 3 weeks pregnant, you're in month 1 of your pregnancy. Only 8 months to go! Still have questions? Here's some more information on how weeks, months and trimesters are broken down in pregnancy.

How big is my baby at 3 weeks?

You ovulated this week, and the moment you’ve been waiting for has finally arrived: You’ve conceived! Meaning your soon-to-be-fetus has started on its miraculous transformation from solitary cell to bouncing baby boy or girl.

Once the winning sperm makes its way through the egg's outer layer, the single-cell fertilized egg — or zygote — immediately forms a barrier to keep other sperm out.

But your zygote doesn't stay single for long. Within hours, it divides into two cells, then four, and so on, until the growing cluster comprises around 100 cells just a few days after that crucial first meeting between sperm and egg. Some will form the embryo, others the placenta, but for now, it’s still just one microscopic ball of cells that’s one-fifth of the size of the period at the end of this sentence. 


Tiny? Yes. But don't underestimate its potential. As it divides, the blastocyst, as it's now called (don't worry, you'll come up with a cuter name soon), travels this week from your fallopian tube to your uterus — a trip that takes about five or six days.

Spoiler alert: Once it arrives during week 4, it will implant itself in the uterine wall and grow for the next nine months. In other words, congratulations! You've got yourself a baby-in-the-making, ready to begin the incredible journey that will end in your arms.

Boy or girl?

So will your lone little cell miraculously become a girl or a boy? Though it will be months before you can find out for sure (if you decide to find out before delivery day), that remarkable determination has already been made, believe it or not.

Ready for a crash course in biology? The fertilized egg contains 46 chromosomes — 23 from each biological parent. The mother always provides an X chromosome, but the father can provide either an X or a Y. If the sperm that fertilizes your egg carries an X, the XX zygote will be a girl. If the sperm is Y-bearing, your XY zygote will be a boy.

Your Body at Week 3

baby size at 3 weeks pregnant

The corpus luteum and pregnancy hormones

For now, it will seem like nothing is happening on the outside — but only for the next couple of weeks. If your timing is right and you've had sex during ovulation, your egg has been fertilized by one lucky sperm and your body is gearing up to host the blastocyst (which will soon become your baby!) that's heading for the uterus, its home for the next nine months.

So what's happening inside this week? Just after the egg is released, the follicle it came from gets a new tenant called the corpus luteum, a yellowish body of cells that occupies the space left by the egg. The corpus luteum starts to produce progesterone and some estrogen, enough of both pregnancy hormones to nourish and support the future baby until the placenta takes over in about 10 weeks.

In the meantime, about a week after fertilization, the blastocyst (or soon-to-be embryo) implants itself in the uterine lining and the placenta starts to take shape. Within six to 12 days after snuggling into the uterus (around week 4 of pregnancy), the cells of the newly developing placenta begin making human chorionic gonadotropin (hCG).

HCG surges during the first trimester before dipping in the second, tells your ovaries to stop releasing eggs and triggers the production of more progesterone and estrogen — which keep the lining of the uterus from shedding and support the growth of the placenta.

As you'll later see, all these hormones play an important role throughout your pregnancy and cause a whole host of body changes, plus symptoms like morning sickness (yay?).

Traces of hCG can be picked up in urine and blood — which explains why home pregnancy tests have you pee on a stick and your OB/GYN runs a blood screening at your first appointment — but you probably won’t get a positive result on a pregnancy test for another week or two.

Pregnancy and sense of smell

Do scents suddenly seem stronger to you than ever? It could be a sign that you're pregnant! A heightened sense of smell is a very real side effect of pregnancy caused by hormones such as estrogen and hCG, which magnify every little fragrance (the good, the bad and the ugly) wafting in the air around you.

Whether it's the food your neighborhood restaurant is cooking up, the garbage on the street corner or your partner's cologne or perfume, your keener-than-ever nose might be picking it up.

The downside of your new superpower? It can ramp up your morning sickness even more. If that's the case, steer clear of the kitchen and local eateries as much as possible, make friends with the microwave (which tends to cause less of a stink) and open the windows.

You can also try washing clothes more often and switching to unscented toiletries. And don't be shy about asking your partner, family and friends to clean up after a workout, go easy on the perfume and brush their teeth after chowing down on that garlicky pasta or onion-loaded burger.

Pregnancy Symptoms Week 3

Lower abdominal pressure
Lower abdominal pressure
Metallic taste
Metallic taste

Tips for You This Week

Up your iron and vitamin C

Toss some berries in your cereal. Adding vitamin C-laden foods to iron-rich fare increases your body's absorption of iron — a nutrient you need to help support your increased blood volume.

You can find vitamin C in fruits and vegetables like kiwis, mangoes, strawberries, melons, bell peppers, tomatoes and asparagus. Iron can be found in soy products, beef, poultry and dried fruit.

Hold off on hair coloring

When it comes to hair coloring, experts agree that safe is better than sorry. So wait until your second trimester (i.e. after week 14 of pregnancy) before getting a touch-up.

When you do go back to the salon, stick to highlights instead of root lightening or root-to-tip color changes (this way, the chemicals won't touch your scalp) and request a gentler coloring option, like an ammonia-free base.

And be aware that hormonal changes can make your hair react differently, so you might not get exactly what you expect, even from your regular formula. Before you do your whole head, see about trying a test strand to be sure.

Try to opt for healthy when eating out

It's easier than you might think to find healthy options when you eat out or order takeout. If you're craving Italian, dine on grilled fish, chicken, veal or lean-beef entrees accompanied by gorgeous greens. Other good choices: pasta and pizza with fresh tomato sauce, pregnancy-safe seafood or cheese.

Enjoy teriyaki fish or chicken, miso soup, edamame and soba-noodle dishes from Japanese restaurants (as well as sushi featuring veggies or cooked fish). And, as long as your stomach's amenable to the spices, Indian restaurants make a particularly nutritious option — order practically anything that isn't fried.

Time to take a pregnancy test?

In the past, you had to miss your period before you took a home pregnancy test (HPT) — then wait a couple of hours before learning the results.

These days, you can find out you're expecting much earlier, faster and with better accuracy than ever before (though accuracy will, of course, get better the closer you are to that missed period). So theoretically you could take one this early if you wanted to!

Still, it can take a week or more after you miss your period before you produce enough of the pregnancy hormone human chorionic gonadotropin (hCG) to be detected on a test. If you're late and negative, retest in a few days.

Choose calcium-rich foods

Calcium helps you and your developing baby build and maintain strong, healthy bones — plus it's essential for heart, nerve and muscle health. If you don’t consume enough calcium during pregnancy, your baby will take it from your bones!

Eat four daily servings (1,000 milligrams) of goodies like Greek or frozen yogurt, calcium-fortified juice or cereal and hard or pasteurized cheese.

Replenish your fluids when you’re queasy

If your morning sickness is accompanied by cramps, fever or diarrhea, you may be dealing with a stomach bug or food poisoning rather than pregnancy nausea.

But whether your stomach is churning from hormones, a virus or that egg salad you had for lunch, the treatment is the same: Rest up and focus on fluids — especially if you're losing them through vomiting or diarrhea.

Drink water, diluted juice (white grape is easiest on the tummy), clear broth, weak decaffeinated tea or hot water with lemon, which can lessen gas. If you can't manage to sip, suck on Popsicles.

Get plenty of protein

Eat three servings of protein daily to help spur new tissue for your baby-to-be. One serving of meat like skinless chicken or lean beef, for example, should be about 3 ounces — roughly the size of a deck of cards. Other great protein sources include eggs, fish, dairy and legumes.

From the What to Expect editorial team and Heidi Murkoff, author of What to Expect When You're Expecting. What to Expect follows strict reporting guidelines and uses only credible sources, such as peer-reviewed studies, academic research institutions and highly respected health organizations. Learn how we keep our content accurate and up-to-date by reading our medical review and editorial policy.

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