4 Weeks Pregnant: Your Symptoms and Baby’s Development

If your baby (now a blastocyst) didn’t implant by the end of last week, it will do so this week. During implantation, the blastocyst burrows into the endometrium, which is the mucous membrane that lines the uterus. Before implantation happens, the blastocyst will break out of its protective covering, and an exchange of hormones will help the blastocyst attach.

This happens when molecules on the embryo’s surface interact with molecules on the uterine wall, which will create a sticky environment that allows the blastocyst to attach—sort of like a rolling rubber ball getting caught in a puddle of honey or syrup.

Your mucus plug will develop

At this point, estrogen stimulates the production of mucus from the glands in your cervix, which will then soften, swell, and pump mucus into the cavity until there are no gaps left. Progesterone causes the mucus to thicken until the cervix seals up. The final mucus plug is about the size of a quarter with about two tablespoons of mucus.

(If you’re interested in tracking your pregnancy week by week—Ava can help.)

How big is my baby?

Baby’s Length: 0.01 in.

Baby’s Weight: 0.04 oz.

How is my baby developing?

Even though your baby is only the size of a poppy seed, a lot of significant developments are already happening.

The cells of the blastocyst now burrowed deep into the wall of your uterus, continue to divide rapidly. The inside of the blastocyst is the embryo; the outside of the blastocyst will later go on to become the placenta.

The placenta releases the hormone human chorionic gonadotropin (hCG), which not only keeps you from releasing additional eggs but also stimulates the ovaries to produce estrogen and progesterone, hormones critical for a healthy pregnancy.

The placenta will soon become your baby’s source of nourishment and waste removal as it channels nutrients and oxygen from your body to your baby and your baby’s waste away from you.

The yolk sac and amniotic sac are also beginning to develop. The fluid-filled amniotic sac will cushion and protect your baby. The yolk sac will provide nourishment until the placenta is ready to take over.

Your baby’s physical features and traits are taking shape

Even more amazing: your baby’s hair color, eye color, sex, and unique family traits like dimples or a widow’s peak have already been determined by the chromosomes you and your partner have provided, although it will be quite a while before they take shape. Your baby even has the basic form of a face, with large dark circles for eyes.

The heart and brain are developing

It’s important to avoid any exposure to toxins right now—and throughout the first trimester—as your baby’s internal organs are going through their initial development.

The embryo continues to grown and now has three distinct layers of cells—the endoderm, mesoderm, and ectoderm—that will develop into specific parts of your baby’s body. The endoderm, the innermost layer, will become your baby’s liver, lungs, and digestive system. The mesoderm will soon be your baby’s skeleton, muscles, kidneys, and heart. The ectoderm, or outermost layer, is on its way to becoming your baby’s skin, hair, eyes, and nervous system.

A sheet of cells has already begun to create the neural tube, where your baby’s brain, spinal cord, and backbone will form. Because this process happens so early in your pregnancy, it’s essential to ensure you’re taking in adequate folate. Folate drastically reduces the likelihood of neural tube defects like spina bifida and anencephaly, which can be disabling or fatal to your baby.

What’s happening in my body?

Next week, in week 5, is when many women start to begin feeling pregnancy symptoms. So now’s a good time to talk about some early signs of pregnancy, as these changes may tip you off even before you get a positive pregnancy test result.

    • Nausea
    • Breast tenderness or soreness
    • Morning sickness
    • Sensitivity to smells
    • Ava data. You may notice some interesting trends in your Ava pregnancy chart. A continuously high pulse rate and skin temperature can—although this isn’t the case for everyone—indicate pregnancy.

Progesterone is increasing

Before jumping to conclusions, however, it’s important to talk about progesterone, because while your levels of progesterone rise in early pregnancy—they are elevated in the second half of a nonconceptive cycle as well. Essentially, many of the symptoms associated with early pregnancy are connected to rising progesterone levels. However, in the two weeks leading up to a normal period, progesterone levels also increase. (Though if you don’t conceive, progesterone levels will fall as the corpus luteum degenerates.)

Many women experience symptoms similar to those of early pregnancy in the weeks leading up to their period. Mood swings, bloating, cramping, fatigue, sore breasts, and even light spotting are also associated with PMS. Again, until you get a positive result on a pregnancy test, there’s no foolproof method for detecting pregnancy.

When can I take a pregnancy test?

Though some women get positive test results in week four, women with longer cycles won’t see a positive test for another week or two. Whatever your normal cycle length is, remember that a pregnancy test will be far more accurate if you wait until your missed period to take it. HCG levels double roughly every two days at this point, and since hCG is the hormone that pregnancy tests detect, waiting even a few days can drastically improve the likelihood that your urine contains enough hCG.

What else should I do?

If you’ve missed your period, take a pregnancy test. Actually, take two! There’s a reason they’re usually sold in boxes of multiples. Even if the first pregnancy test comes back positive, wait two days and take another to be sure your result wasn’t the false positive that can come with a chemical pregnancy.

The same goes if your first pregnancy test comes back negative, as you might be so early in your pregnancy that levels of hCG have not had a chance to rise to detectable levels.

Once your pregnancy test comes back positive, it’s a good time to call your doctor or OB-GYN. Chances are, your doctor won’t schedule an appointment until your 8- to 12-week mark, as this is when they’ll be able to estimate your due date using an ultrasound.

Some other things to consider:

  • Go to bed early or sneak in extra sleep where you can. Sleep disturbances can start early in pregnancy, so make sleep a priority before it gets more difficult.
  • Take your mind off things by listening to music, spending time with friends, or reading a good book.
  • Keep taking your prenatal vitamin! Since your baby’s neural tube is now forming, having an adequate supply of folate is more crucial than ever.
  • Now is an excellent time to start considering what kind of doctor or midwife you really want.

Smoking and pregnancy—know the risks

If you’ve been a smoker in the past, hopefully, you’ve already given up. Smoking during pregnancy has been linked to numerous issues, including earlier puberty, allergies and asthma, cancer risks for daughters, heart defects, and poor lung function.

Sometimes, though, giving up smoking yourself isn’t enough. Recent research indicates that even secondhand smoke can negatively affect fetal brain development. The researchers found that smoke exposure damaged the parts of the brain connected with learning, memory, and emotional responses. It might be a good time to ask your friends and family to butt out when they’re around you. Exposure to secondhand smoke may also increase your risk of miscarriage, stillbirth, ectopic pregnancy, and low birth weight.

This Week’s FAQs

What is the best pregnancy test?

The first signs of pregnancy—what will I feel, and when?

When does implantation happen?

How do I cope with the waiting?


Lindsay Meisel

Lindsay Meisel is the Head of Content at Ava. She has over a decade of experience writing about science, technology, and health, with a focus on women's health and the menstrual cycle. Her work has been featured on The Fertility Hour, The Birth Hour, The Breakthrough Journal, and The Rumpus.

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