In week 4 of pregnancy, your body is beginning to form the placenta and amniotic sac.

Symptoms like abdominal pressure and tender breasts may appear this week, and as the cluster of cells that will soon become your baby burrows into your uterine lining, you may also spot some implantation bleeding. (But if you don't notice any symptoms at all yet, that's completely normal too.)

Your Baby at Week 4

At a Glance

Two sets of cells
Two sets of cells
Your itty bitty embryo has two layers of cells called the epiblast and the hypoblast. Soon they’ll develop into all of your baby’s body parts and systems.
Seeing double
Seeing double
You normally don't have your first ultrasound until at least week 6, but you might be able to spot the gestational sac as early as halfway through week 4 — and if you're having twins, you'll likely see two of them!
Baby's yolk sac
Baby's yolk sac
Before the placenta forms, you develop a yolk sac, which can be visible next week. This sac produces blood and helps to nourish your young embryo.

4 weeks pregnant is how many months?

If you’re 4 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 4 weeks?

Despite its extremely tiny size — no longer than 1 millimeter and no bigger than a poppy seed (think about that as you eat your morning bagel) — your little embryo is busy setting up house.

Placenta and embryo begin forming

While you may have just started to wonder whether you're pregnant, your soon-to-be baby has already found its home: The blastocyst has completed its journey from your fallopian tube to your uterus.

Once there, it burrows into your uterine lining and implants — making that unbreakable connection to you that'll last the next eight months (and a lifetime after that).

As soon as that little ball of cells is settled in your uterus, it will undergo the great divide, splitting into two groups. Half of what's now called the embryo will become your son or daughter, while the other half forms the placenta, your baby's lifeline — which channels nutrients and carries waste away until delivery.

Development of the embryo and amniotic sac

While the amniotic sac (also called the bag of waters) forms around it, so does the yolk sac, which will later be incorporated into your baby's developing digestive tract.

The embryo now has three distinct layers of cells that will grow into specialized parts of your baby's body. The inner layer, known as the endoderm, will develop into your baby's digestive system, liver and lungs. The middle layer, called the mesoderm, will soon be your baby's heart, sex organs, bones, kidneys and muscles. And the outer layer, or ectoderm, will eventually form your baby's nervous system, hair, eyes and outer layer of skin.

Your Body at Week 4

baby size at 4 weeks pregnant

No symptoms yet?

Just a week after fertilization, baby-making is still in its infancy, so to speak. At 4 weeks pregnant, your body is gearing up, big-time — transforming from a tried-and-true buddy to a weird and wacky science experiment.

Chances are you're oblivious to all the hubbub. While some women experience those pesky, PMS-like early pregnancy symptoms, including mood swings, bloating and cramping, others don't feel a thing.

Whatever you're feeling or not feeling, it may be too early to see a reliable result on your pregnancy test.

The egg implants

Though you may not feel pregnant quite yet, here's what's going on behind the scenes.

The fertilized egg and your uterus are making contact this week in a process called implantation, as the blastocyst you'll one day call your baby begins to attach itself to the uterine lining.

Up to 25 percent of the time, implantation bleeding will occur as that bundle of cells burrows its way into the uterine wall. Implantation bleeding, which is usually very scant and either light pink, light red or light brown, occurs earlier than your expected period. 

Don't mistake it for your period and don't worry about the bleeding — it's not a sign that something is wrong. You might feel a little pressure in your abdomen (nothing to worry about!) and your breasts may feel a little tender and become even bigger (but get ready for more growth spurts!).

Pregnancy hormones will soon make an appearance

Within six to 12 days after fertilization, the egg starts to release hCG, or human chorionic gonadotropin — the pregnancy hormone that will very soon turn that line on your pregnancy test pink or blue and your world upside down.

HCG alerts the corpus luteum (the once-follicle this egg was released from) that it needs to stick around and produce progesterone and estrogen to nourish the pregnancy until the placenta takes over about eight weeks from now.

Pregnancy Symptoms Week 4

Implantation bleeding
Implantation bleeding
PMS-like symptoms
PMS-like symptoms

Tips for You This Week

Don't forget vitamin D

Most of your vitamin D supply comes from the sun or fortified milk. If you don't drink the white stuff, you'll need to find your D from other sources.

That's because vitamin D is essential for maintaining healthy teeth and bones, and helps your body absorb calcium (and you already know why you need to absorb plenty of that).

You can find vitamin D in many prenatal vitamins as well as from fortified milk, fortified orange juice and egg yolks. Talk to your doctor about how much you need (600 IU is the standard recommendation among the expectant set, but some may need 1,000 IU or more).

Find your due date

Think you need a Ph.D. in quantum physics to figure out your due date? Doing the math is actually a lot easier than you think — even if you slept through high school algebra. Your estimated due date is 40 weeks from the first day of your last period.

Here's the slightly confusing part: If you do give birth on that day, your baby will have clocked in only 38 weeks in utero, not 40. That's because the pregnancy countdown begins two weeks before your baby is even conceived — making you about four weeks pregnant before you see a positive pregnancy test.

Regardless, you shouldn't plan your schedule around your estimated due date. After all, it is just an estimate. Most babies are born between 38 and 42 weeks, and babies of first-time moms are more likely to arrive on the later side. Only a handful of babies actually make their debuts right on schedule.

Learn more about how to calculate your due date.

Healthy fats are good fats

Your baby needs some fat — especially essential fats like omega-3 fatty acids. DHA, one of those omega-3s, is a major component of the human brain and retina and it’s important for your baby's developing brain and eyes.

You can get DHA from pregnancy-safe fish such as trout and wild salmon and from DHA-fortified eggs. If you can't stomach fish, you can also find DHA in prenatal vitamins and supplements, including a vegetarian DHA made from flaxseed.

Avoid secondhand smoke

You may not smoke, but if those around you do, there could still be risks to baby. Recent research has found that exposure to secondhand smoke may increase your risk of miscarriage, low birth weight, ectopic pregnancy and other complications. So try to steer clear as much as possible.

Try a new comfort food

Often, what starts out as a comfort food (meaning one of the few things you can stomach) later becomes associated with nausea — and actually starts to trigger another round of the queasies because you’ve eaten so much of it.

If you’re so sick of saltine crackers that they’re actually beginning to make you sick, for example, switch to another comforting carb instead.

Make a prenatal appointment

If you haven't already, now's the time to make your first doctor's appointment! Some OB/GYNs ask that you wait until you’re at least 6 to 8 weeks pregnant before a visit, but since early prenatal care is so important, it's best to get something on the calendar as soon as you have a positive pregnancy test.

Make sure to research the kind of doctor or midwife you really want first.

Stave off food poisoning

You may be eating for two (or, more accurately, one and a fraction) these days, but your menu just got a whole lot smaller. The reason for cutting out sushi, runny eggs and raw cookie dough batter? To safeguard against foodborne illnesses like listeria and salmonella.

Some of the most common food poisoning culprits are undercooked proteins like poultry, eggs, meat or fish, and raw dairy or juices.

On the off chance that you are sickened by food poisoning from salmonella, the bug will likely have to run its (unpleasant) course. But don’t worry, because it's unlikely that your baby will be in any danger.

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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