4 Weeks pregnant: Baby development, symptoms and signs

Your body at week 4

Now that you’ve received a positive pregnancy test, it means that implantation has occurred: your fertilized egg has successfully attached itself to the lining of your uterus, where it will grow and develop for the next eight months. This microscopic chain of events marks the official start of your pregnancy, and sets off a cascade of hormonal shifts. 

Among these is the production of human chorionic gonadotropin (hCG), which is the hormone at-home pregnancy tests detect. These rising hCG levels are responsible for the early pregnancy symptoms you may be feeling. However, it’s normal not to experience any symptoms just yet [1,2, 3].  

Understanding the timing of pregnancy

When you hear “4 weeks pregnant,” you may be confused — which is totally normal. This is because the timing of your pregnancy is measured according to your menstrual cycle, rather than the date of conception.

Specifically, your pregnancy is calculated from the first day of your last menstrual period (LMP) [4]. This means by the time you receive a positive pregnancy test, you're already considered about 4 weeks pregnant, even though the fertilized egg likely implanted in your uterus just a few days ago.

In other words, your pregnancy is just beginning on a biological level, but medically, you're already one month in. Consider it two “bonus” weeks on the journey ahead: 4 down, 36 to go! 

Pregnancy symptoms at week 4

At week 4, a wave of hormonal changes can bring on early pregnancy symptoms. However, it’s also perfectly normal if you're not experiencing any symptoms at all. 

Fatigue
Feeling unusually tired, sluggish, or drained? You’re not alone — growing a human is nothing short of exhausting. Fatigue is one of the most common symptoms of early pregnancy, and at 4 weeks, you might feel like you can’t get through the day without a nap (or two). This heavy, worn-down feeling is a result of shifts in your hormones, including a rise in progesterone levels. 

In addition, your body is producing more blood to support your pregnancy, which can lead to lower blood pressure and decreased energy. If you’re feeling wiped out, listen to your body and give yourself the rest you need [5, 6].

Nausea or vomiting
An upset stomach is a hallmark symptom of the first trimester — 70% of pregnant people experience it!— and for some, that first wave of nausea is what makes their pregnancy feel “real” [7]. Nausea and vomiting during pregnancy are referred to as morning sickness, though the term is a misnomer, and these symptoms can occur at any time of day. 

At 4 weeks pregnant, you may find yourself feeling queasy, nauseated, and  sick to your stomach, which may lead to vomiting. However, if these symptoms become more severe (i.e. you cannot keep food down), it’s important to speak to your doctor. This could indicate a rare condition called hyperemesis gravidarum (HG) [6, 8].

Mood swings
If you’re feeling more emotional than usual — teary one moment, frustrated the next, or just not quite yourself — it’s because mood swings are common in early pregnancy. Part of this emotional whirlwind stems from the fact that you’re processing such a massive life change, but your brain chemistry is at play here, too. Remember those shifting hormones? These fluctuations have an effect on the neurotransmitters in your brain, which are responsible for regulating your mood.  

It’s completely normal to experience emotional highs, lows, and everything in between. Remember to be gentle with yourself, and lean on loved ones or professionals for support [9, 10]. 

Cramping and light bleeding
Even though you’ve said goodbye to your period for the next nine months, you may get a visit from a common PMS symptom.This pain is typically mild or moderate, and some describe a tingling, prickling, and pulling sensation that’s different from the dull pain of menstrual cramps. Implantation cramping may be accompanied by light bleeding, referred to as implantation bleeding [11]. It's important to note that there is currently no strong scientific evidence for this phenomenon. Implantation bleeding is thought to be caused by the fertilized egg cell coming into contact with blood cells in the uterus lining as it implants into the uterine wall. [12].

If you are experiencing bleeding, it’s important to speak with your doctor or midwife to eliminate causes for concern. Learn more about early pregnancy bleeding. 

Your baby’s development at week 4

At week 4 of pregnancy, you may not feel much — but incredible changes are already happening within. From your recently developed embryo to the start of a totally new organ, incredible things are happening inside your uterus. 

Your baby’s size at week 4

At this point in your pregnancy, the embryo is roughly the size of a poppy seed (0.078 inches, or 1-1.98 mm, long) and doesn’t weigh anything yet.  But don’t be fooled by its tininess. It’s growing and developing at high speed [13].

Important terminology at week 4 of pregnancy

There’s so much going on inside your body, and you may feel like you’ve encountered a whole new world of language at the same time. We’ve broken down some of the common terms that are important to understand at this point in the journey, and how they relate to your pregnancy.

Implantation
At 4 weeks pregnant, your body has completed the process of implantation. This means the fertilized egg has successfully made its way through the fallopian tube and attached to the lining of your uterus [3]. Technical terms aside, this is an important hurdle in the conception journey and a crucial first step to growing your little one. 

Blastocyst
This is a monumental week in your baby’s early development. However, the more accurate description of your little one would not be a baby, but a ball of cells, called a blastocyst [14]. At this stage of pregnancy, the cells of the blastocyst divide, develop, and transform into the building blocks of new life [15].

Embryo
From the inner cells of the blastocyst, the embryo is formed. This is the tiny structure that will eventually be your baby. By week 9 of pregnancy, the embryo becomes a fetus [15], but for now, your embryo is made up of three cell layers that form the beginnings of your baby’s body [16]. The innermost layer develops into their lungs and stomach, while the middle layer becomes their heart, muscles, and bones. The outer layer is what forms the brain and the structures in their eyes, teeth, skin, and nails [16].

Placenta & amniotic sac
In addition to the embryo itself, your body is developing key structures to protect and nourish it.

Remember that ball of cells, known as a blastocyst? The outer cells of the blastocyst begin forming what will become the placenta. This is responsible for nourishing your baby throughout pregnancy [16]. This organ — yes, your body grows a whole new organ — will supply your baby with the oxygen, nutrients, and hormones it needs to thrive [3].

Another key structure in your pregnancy is the amniotic sac, which forms around the embryo during this time. Filled with liquid (known as amniotic fluid), this structure protects your baby and helps regulate their temperature. In other words, it keeps your little one safe and warm [17]. 

4 weeks pregnant checklist

Calculate your due date: “When is my baby due?” is often the first question people ask after receiving a positive pregnancy test. At four weeks pregnant, you can calculate your due date by counting 40 weeks from the first day of your last menstrual period [4]. If you’re unsure of that date or have irregular cycles, don’t worry. Your provider will likely use an early ultrasound to confirm the timeline. 

Keep in mind that this date is only an estimate, and very few babies arrive exactly on their estimated due date. If you’ve been using Natural Cycles to plan your pregnancy,  the app will automatically calculate your due date (along with many other features in NC° Follow Pregnancy). Keep in mind, this might look a little different than the due date provided by a healthcare professional, as this is based on the ovulation date. 

Schedule your prenatal appointment: Now is the perfect time to call and schedule your first appointment with your preferred OB-GYN or midwife. This will take place around 8 weeks of pregnancy, though the timing of your first visit may vary based on your medical history or your medical condition [18]. This first appointment is often longer and more in-depth (as compared to routine visits later in pregnancy), and may include a physical exam and a review of your medical history, any medications you're taking, and lab work. It’s also a great opportunity to ask questions, talk about your lifestyle or symptoms, and get clear on what to expect in the weeks ahead.

Follow pregnancy-safe guidelines
Now that you’re officially expecting, it’s important to create a healthy environment for your early pregnancy. This means avoiding foods and substances that are considered pregnancy-unsafe (such as unpasteurized cheese and high-mercury fish, or pesticides and fertilizer); and making lifestyle changes as necessary (such as cutting out alcohol and smoking). It’s also best to speak with your doctor about any over-the-counter and prescription medications you’re taking. Adhering to these best practices helps support both your health and your baby’s development. [19].

Tips for 4 weeks pregnant

At 4 weeks pregnant, you’re stepping into a brand-new chapter—and your body is rising to the occasion. This week is all about grounding yourself, taking care of your body, and learning more about the changes to come. Here are some small but meaningful ways to support your health and well-being:

1. Eat nutritious foods — and smaller meals if necessary: Now that you’re officially expecting, nourishing your body becomes even more important. Prioritize nutrient-dense foods like whole grains and lean proteins, along with foods that are high in vitamins like folate and iron (such as leafy greens) [20, 21].  If you’re feeling nauseous, try eating smaller, more frequent meals made up of simple, starchy foods — such as bread, rice, or crackers — which can be easier on your stomach [22]. Learn more about what to eat when pregnant.
2. Continue taking your prenatal vitamins. It’s recommended that you begin taking prenatal vitamins before you start trying for a baby. Continue taking your vitamins as directed by your doctor, and if you haven’t started, now is the time to start. Learn how to choose the right prenatal vitamin for you [20]. 
3. Prioritize rest & relaxation: Your body is working overtime, so it’s important to give yourself permission to slow down. This could mean going to bed earlier, carving out time for an afternoon nap, or simply saying no to commitments that drain you. Mental rest is important, too. Finding ways to calm your mind (whether journaling, meditation, or prenatal yoga) can help ease anxious thoughts and create space for emotional grounding.
   

Following your pregnancy with Natural Cycles

Natural Cycles is the first app of its kind to be FDA Cleared for use as contraception but did you know you can also use the app for more than just birth control?Natural Cycles can be used throughout pregnancy and postpartum, too. NC° Follow Pregnancy provides prenatal guidance, comprehensive tracking, and seamless support for your pregnancy. You can access weekly guides like this one, monitor your symptoms, your baby’s movement, and more. Wherever you are in your journey, find out how Natural Cycles can work for you.