4 weeks pregnant: Your guide to this week of your first trimester

    Updated 29 June 2023 |
    Published 24 February 2019
    Fact Checked
    Reviewed by Dr. Nazaneen Homaifar, Obstetrician and gynecologist, Inova Health System, Washington, DC, US
    Written by Kate Hollowood
    Edited by Sarah Biddlecombe
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    From a missed period to tender breasts, here’s the lowdown on being 4 weeks pregnant.

    This week can be an anxious and exciting time, as it may be the week that you can take a pregnancy test. If you discover that you’re pregnant, you’ll technically be 4 weeks pregnant. This is because your gestational age is calculated from the first day of your last period (in comparison, the fetal age is calculated from fertilization).  

    You’re no doubt full of questions about the changes happening to your body and your newly growing baby. While it’s still very early days, it can be reassuring to know about the symptoms to look out for and how you might feel at this stage. Keep reading to learn about early pregnancy symptoms and get some advice from a Flo expert about what else to expect at 4 weeks pregnant.  

    Your baby at 4 weeks pregnant

    Forming lungs 

    Even though they’re only the size of a poppy seed, amazingly, this week your baby is starting to form lungs. By 26 weeks, their lungs will be able to inflate and deflate. However, they won’t be able to function fully by themselves until 36 weeks

    Benefitting from a growing placenta 

    Your baby is also benefiting from your growing placenta when you’re at 4 weeks. This will be fully formed by 12 weeks, although it will keep growing throughout your pregnancy. From 12 weeks onward, it will take over the transfer of nutrients to your baby. For now, your baby is getting most of their nourishment from a tiny yolk sac. This might sound strange, so let us explain. Essentially, the yolk sac is a structure outside of the baby, located within the surrounding gestational sac in the uterus. The yolk sac delivers nutrients from you to your growing baby, produces cells that turn into important structures, and circulates gasses between the two of you. Clever, huh? 

    How big is a baby at 4 weeks?

    Length (crown to rump): Around 0.08 in. (2 mm)

    Weight: Too small to calculate accurately 

    Size: Equivalent to a poppy seed 

    All measurements are approximate and vary within the normal range.

    Your body at 4 weeks pregnant

    4 weeks pregnant symptoms

    If you have a regular menstrual cycle, 4 weeks is when you might experience a missed period — one of the very first pregnancy symptoms. The average menstrual cycle is 28 days long, but anything between 21 and 35 days is considered regular. Wondering how long your individual menstrual cycle is? Don’t forget, you can track your periods (and figure out when you might have missed one!) using an app like Flo.

    And why is a missed period such an important first sign that you might be pregnant? You may know the science behind this already, but just in case, let’s recap:

    • With every menstrual cycle, your body prepares the uterus to support a potential baby
    • Rising levels of the hormones estrogen and progesterone cause the uterine walls to thicken, readying them to nourish a fertilized egg
    • When fertilization doesn’t happen, your body sheds this uterine lining, along with blood and mucus, from the vagina and cervix, resulting in the bleeding you know as your period. 
    • But if fertilization does occur, then the fertilized egg will implant itself into your uterus, meaning you’re pregnant — and you won’t have a period.

    So if you’ve missed a period, then that’s your cue to take a pregnancy test. “The best time to take a urine pregnancy test is at the time of the missed period,” says Dr. Jenna Flanagan, academic generalist obstetrician and gynecologist, Beth Israel Deaconess Medical Center, Massachusetts, US. “Taking the test before this time can lead to a false negative, as the hCG [human chorionic gonadotropin — a hormone that your body only produces during pregnancy] hormone levels have not risen to a level that is detectable in the urine.” Your doctor may also recommend doing a different type of pregnancy test. “A serum/blood pregnancy test can be taken a few days earlier, as it is more sensitive,” adds Dr. Flanagan. 

    You may not notice any other first-trimester symptoms at this stage, although some women do get tender breasts at 4 weeks pregnant. 

    “The pregnancy hormone hCG is often the cause of many early pregnancy symptoms — such as nausea, vomiting, and gastrointestinal upset — but it is not very high in the fourth week,” explains Dr. Flanagan. “Therefore, many women do not experience any symptoms [at this stage]. However, other hormones, such as progesterone, are rapidly increasing, which can cause symptoms like breast tenderness and bloating. Additionally, some people may experience mild or vague cramping sensations as well.”

    So the key thing to remember is that if you feel the same as usual, then that’s nothing to worry about, as that is actually very common at this point. 

    Dr. Flanagan continues, “While some women report pregnancy symptoms as early as 4 weeks, more commonly they begin in earnest at 6 weeks and beyond due to the rising levels of hCG. This can cause nausea, vomiting, and increased urination. Meanwhile, rising progesterone levels cause the breast tissue to grow, [which can cause] soreness of the breast tissue, as well as constipation and bloating.”

    Want to know more?

    Download the Flo app for tailored insights throughout your pregnancy

    Your questions answered

    What should I be feeling at 4 weeks pregnant?

    There’s no right or wrong way to feel at 4 weeks pregnant. It’s still very early days, and you may not have any pregnancy symptoms yet — or even a positive pregnancy test! 

    If you’ve noticed a missed period, you might be feeling all kinds of different emotions, from happiness and excitement to worry and fear. Rest assured that all of these feelings and more are totally valid and normal. 

    You may also be wondering when to tell people you’re pregnant. If this is the case, know that you shouldn’t feel any pressure to tell anyone until you’re ready. At the same time, being able to talk through how you’re feeling with someone you trust can help you to navigate the start of your pregnancy journey. 

    What does it mean to be 4 weeks pregnant?

    At 4 weeks, you are officially pregnant. You’ve technically been pregnant since implantation, when your fertilized egg burrowed into your uterus at around 3 weeks pregnant. But as 4 weeks is when you may experience a missed period, this is the week you might take a test and find out that you’re pregnant. 

    How does your stomach feel at 4 weeks pregnant? 

    Your baby is only the size of a poppy seed when you’re 4 weeks pregnant, so you’re not going to notice a pregnancy bump just yet. For some parents, a baby bump will start to show from around 12 weeks

    Your 4 weeks pregnant checklist

    Have you heard about the benefits of vitamin B6 during pregnancy? This nutrient plays a critical role in the development of your baby’s red blood cells. Not only that, but it has benefits for you too, as it can help to ease nausea

    You don’t necessarily need to start taking vitamin B6 as a supplement, as there are plenty of ways to get it through your diet. Tuna, salmon, chickpeas, and dark leafy greens are some of the foods with the highest vitamin B6 content. If you do want to start taking a B6 supplement, remember that it’s always a good idea to check with your doctor before taking any new supplement while you’re pregnant.

    Adding these foods into your diet is one of a variety of lifestyle changes you might want to make during your pregnancy. For example, it’s a good idea to start limiting your caffeine intake to 200 mg a day, which is equivalent to a 12-oz. cup of coffee. Experts also recommend that you should stay away from alcohol, smoking, and recreational drugs throughout your pregnancy. 

    When to consult a doctor at 4 weeks pregnant

    Once you’ve got a positive pregnancy test, it’s time to make your first prenatal appointment with your doctor, which is sure to be an exciting moment. To find out how to prepare for this, have a look at our checklist for your first prenatal appointment

    Wondering what you can expect from this appointment? At your first visit, your doctor may talk you through your medical history, take various blood tests (to check for things like your blood type or conditions like anemia), and recommend lifestyle choices for your pregnancy.    

    Dr. Flanagan notes that if you have had an ectopic pregnancy in the past, your doctor may also want to monitor you more closely to make sure you’re not having another. This type of pregnancy is when the fertilized egg implants outside of the uterus, usually in either one of the uterine tubes. 

    “Typically, ectopic pregnancies do not cause hCG to rise appropriately,” says Dr. Flanagan. “So the monitoring typically includes a series of hCG blood tests to ensure the hCG level is rising regularly. Once the hCG levels are known, arrangements can be made to schedule an early ultrasound to document the location of the pregnancy.” 

    Experiencing an ectopic pregnancy can be heartbreaking. If you’ve experienced one and are pregnant again, it’s natural to feel particularly anxious during these early weeks. Know that your doctor is there to look out for you and that you are definitely not alone. Ectopic pregnancies affect around 1 in 100 pregnancies, and for most people, they do not happen again.

    Of course, any type of pregnancy loss can be devastating. But if you’ve previously experienced a miscarriage that wasn’t caused by an ectopic pregnancy, you’ll be monitored as normal. This can be hard to accept — wanting extra tests and reassurance in this case is understandable. 

    If it helps, hold on to the fact that most people go on to have a healthy pregnancy after miscarriage, so there’s good reason to be hopeful. Be extra kind to yourself if you’re feeling worried and don’t hesitate to speak to your doctor if you have any concerns. Plus, remember that you don’t need to wait until your appointment if you have any concerns or questions about your pregnancy. However, at 4 weeks pregnant, you should contact your doctor immediately if you experience: 

    This isn’t an exhaustive list and just an example of some of the changes you should look out for. Some of these can be a sign of miscarriage or other health complications, so it’s essential that you speak to your doctor about the best next step for you. And if you’re ever worried about any other symptoms you experience during pregnancy, then don’t hesitate to reach out to your health care provider. 

    4 weeks pregnant: The takeaway

    When you’re 4 weeks pregnant, it can be a significant milestone in your pregnancy journey, as it’s the time when you might be able to take a test and find out you’re officially pregnant. It’s also the first week you might experience any pregnancy symptoms. Because it’s still early days, these are most likely to be a missed period and tender breasts. Remember that it’s also perfectly normal to feel no different than usual this week. However you’re feeling, try to take things easy and give yourself plenty of time to let your pregnancy news sink in.


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    History of updates

    Current version (29 June 2023)

    Reviewed by Dr. Nazaneen Homaifar, Obstetrician and gynecologist, Inova Health System, Washington, DC, US
    Written by Kate Hollowood
    Edited by Sarah Biddlecombe

    Published (24 February 2019)

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