An ultrasound is a routine part of prenatal care at six to nine weeks. The ultrasounds we might do prior to that, and the information those exams would reveal, generally occur in four stages:
- Stage One: If performed around the time a women’s menstrual period is expected, this ultrasound typically shows a fluffy, thick lining of the uterus that’s preparing for the fertilized egg to implant.
- Stage Two: This is usually at four to five weeks after a pregnant woman’s last period. The ultrasound commonly shows a small collection of fluid within the lining of the uterus that represents the early development of the gestational sac.
- Stage Three: This is usually about five and a half weeks after a pregnant woman’s last period. The ultrasound typically shows a gestational sac and within it we can see a 3-5 mm bubble-like structure, which is the yolk sac.
- Stage Four: Approximately six weeks after a pregnant woman’s last period, we can see a small fetal pole, one of the first stages of growth for an embryo, which develops alongside the yolk sac.
While these are the expected times to see the developing pregnancy with an ultrasound, not all pregnancies develop along the same timeline. Just like pregnancy tests, if there’s variability in the length of the menstrual cycle or when fertilization takes place, then what we see on ultrasound can change. It’s important that this ultrasound is performed vaginally for high-quality pictures.
If the first ultrasound doesn’t show a developing baby with a heartbeat, when should the next one be scheduled?
Newly pregnant women get anxious if we don’t see both a fetus and a heartbeat on the first ultrasound and frequently want to come back soon after for another look. But it takes time to move through the early stages of pregnancy. And repeating an ultrasound still won’t be able to reassure the patient that the fetus is alive and growing, if we do it too soon.
The general recommendations are to wait two weeks if we only see a gestational sac and at least 11 days if a gestational and yolk sac are seen without a fetal pole. I prefer to wait two weeks for the next ultrasound in both of these scenarios. If we can see an early fetal pole but can’t see cardiac movement, then we repeat an ultrasound in one week. I know waiting is hard – but in my experience, it is much better to wait and get a definitive report on the status of your pregnancy than potentially have to come back multiple times.
Pregnancy loss can occur during any stage of pregnancy, though it’s most common in the first trimester. If we do an ultrasound and the length of the baby is more than 7mm, we should always see movement of the fetal heart. If we don’t, we know the pregnancy is not going to develop.
We believe you should always know the source of the information you're reading. Learn more about our editorial and medical review policies.on September 30, 2022
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See moreExplore Pregnancy Week 4
- Baby development
- Body changes
- Common symptoms
- Tips for you
- Recommended products
- Birth month group discussions
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
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
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
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
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
If you find that you’re spotting this week — right around the time that you would have had your period or a little before — don’t be alarmed. This is usually a sign that the embryo has implanted itself into the uterine wall. A good thing! Don’t have any spotting at all? Don’t worry, either — less than one quarter of women experience implantation bleeding, so not having this symptom doesn’t mean that you’re not pregnant. Read More
PMS-like symptoms
You've never been so affected by these hormones-on-a-rampage coursing through your body, which are causing everything from moodiness and bloating to mild cramping. Get familiar with pregnancy hormones now...you'll be hearing a lot about them over the next nine months or so. Read More
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.
View Sources
- What to Expect When You’re Expecting, 5th edition, Heidi Murkoff.
- WhatToExpect.com, Eating Fish During Pregnancy: What Varieties Are Safe?, June 2020.
- WhatToExpect.com, Eating Vegetarian and Vegan During Pregnancy, June 2020.
- WhatToExpect.com, Toxoplasmosis During Pregnancy, March 2021.
- WhatToExpect.com, What to Do About Bloating During Pregnancy, February 2021.
- WhatToExpect.com, Exercises to Avoid During Pregnancy, December 2019.
- WhatToExpect.com, Pregnancy Nutrition Chart: 33 Essential Nutrients for Pregnant Women,
June 2020.- WhatToExpect.com, Are E-Cigarettes and Vaping Safe While Pregnant?, February 2021.
- WhatToExpect.com, Your Guide to Prenatal Appointments, February 2021.
- WhatToExpect.com, Listeria Exposure During Pregnancy, September 2020.
- WhatToExpect.com,Salmonella During Pregnancy, October 2020.
- American College of Obstetricians and Gynecologists, How Your Fetus Grows During Pregnancy, August 2020.
- National Institutes of Health, National Library of Medicine, Fetal Development, June 2021.
- National Institutes of Health, Eunice Kennedy Shriver National Institute of Child Health and Human Development, How Do I Know If I’m Pregnant?, January 2017.
- National Institutes of Health, National Library of Medicine, The Basics of Brain Development, December 2010.
- Society for Endocrinology, Hormones of Pregnancy and Labour, March 2018.
- American College of Obstetricians and Gynecologists, Early Pregnancy Loss, February 2020.
- National Institutes of Health, National Library of Medicine, Pickles and Ice Cream! Food Cravings in Pregnancy: Hypotheses, Preliminary Evidence, and Directions for Future Research, September 2014.
- University of Rochester Medical Center, First Trimester Fatigue, 2021.
- American College of Obstetricians and Gynecologists, Morning Sickness: Nausea and Vomiting of Pregnancy, May 2020.
- UpToDate, Treatment and Outcome of Nausea and Vomiting of Pregnancy, May 2021.
- National Institutes of Health, National Library of Medicine, Ptyalism Gravidarum, November 2009.
- Centers for Disease Control and Prevention, Toxoplasmosis: Pregnancy FAQs, September 2020.
- American College of Obstetricians and Gynecologists, Oral Care During Pregnancy and Through the Lifespan, 2017.
- American Dental Association, Is It Safe to Go to the Dentist During Pregnancy?
- FoodSafety.gov, People at Risk: Pregnant Women, September 2020.
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