Welcome in, baby! Your baby has finished the trip from the fallopian tube to your uterus and is implanting him or herself into your uterus. Here’s what to expect in pregnancy week four.

Baby at 4 weeks pregnant

After it has attached itself to your uterine lining, that ball-of-cells baby of yours begins to divide in two. Half will become your baby girl or boy, and the other half has been tapped to become the placenta, an organ that will grow around the baby and nourish him or her for the remainder of the pregnancy.

Your baby, currently known as an embryo, has three layers, and each is beginning to grow into specialized parts of the body. The endoderm, or inner layer, will become your baby’s digestive system, liver, and lungs. The mesoderm, or middle layer, will become your baby’s heart, sex organs, muscles, bones, and kidneys. The outer layer is called the ectoderm and will become the nervous system, hair, skin, and eyes. 

Also underway is the formation of the amniotic sac, or bag of waters, that will provide a suspension system of sorts for your baby. The yolk sac, later to become part of baby’s developing digestive track, is also forming. 

Your body at 4 weeks pregnant

One in four women experiences some bleeding or spotting during implantation, when the fertilized egg burrows into the uterine wall 6 to 12 days after conception. Implantation spotting or light bleeding might last for a few hours or days and is not a sign that anything is wrong.

If you expected your period this week and are still waiting, you could take a home pregnancy test and get very accurate results: there’s 99 percent chance of a correct response on the first day your period is late. That’s because the newly developing placenta begins producing the pregnancy hormone human chorionic gonadotropin (hCG) within days of implantation. It’s the presence of this hormone in your urine or blood that triggers a positive pregnancy test.

Determining when you actually conceived isn't an exact science. You've only been pregnant for two weeks by the time you miss your period, but doctors calculate pregnancy based on the date of your last menstual period (LMP). That’s why you’re considered four weeks pregnant at this point.

Four weeks pregnant ultrasound

It’s very early in your pregnancy and that makes it hard to detect by ultrasound. It is possible that a gestational sac could be seen via transvaginal ultrasound in the second half of this week.

How big is my baby at 4 weeks pregnant?

At 4 weeks pregnant, your baby is the size of a poppy seed, on average measuring at 0.8 inches long and weighing on average at 2 milligrams.

Four weeks is how many months pregnant? 

At this stage in your pregnancy, you're one month (1m0w) pregnant. 

Common symptoms at 4 weeks pregnant

Some pregnancy symptoms you might be experiencing at four weeks pregnant:

• A missed menstrual period. This is often the most obvious first indication that you are pregnant. If you are pregnant, a home pregnancy test will almost certainly show a positive result on the first day your period is late.
• Breast tenderness. Your breasts may be starting to become fuller and you may discover that your bras are fitting a bit snugger.
• A heightened sense of smell. Pregnancy hormones can kick your sniffer into high gear, for better or worse. And smells that didn’t bother you before might be extremely off-putting when you have a baby on board!
• Headaches. Pregnancy hormones can cause pretty killer headaches in some women almost right away. 
  Shortness of breath. This symptom can crop up pretty early on in a pregnancy because the fetus needs more and more oxygen as it develops and grows. And unfortunately, this one will probably stick with you through all nine months. 
• Metallic taste in your mouth. Another symptom to blame on pregnancy hormones, which can make your taste buds go all kinds of crazy.
• Mild abdominal cramping. There’s a lot going on in there as your body sets up for nine months of baby growing, so you might begin feeling some mild pains that ironically indicate all is well. Increased blood flow, thickening of the uterine wall, and embryo implantation in the uterus in week four can all cause mild aches, pains, and cramps. 
• Feeling unusually tired or even exhausted. As your body works to grow another human, you may begin to feel the effects in the form of extreme exhaustion.

For most women, classic early pregnancy symptoms like fatigue, nausea, and rollercoaster emotions begin in the sixth week of pregnancy.

Pregnancy checklist at 4 weeks pregnant 

Getting a positive pregnancy test is truly a life changer. One thing it does is provide a timeline: in nine months you’ll be holding your tiny new baby. Prep for that moment begins in earnest now.

With a baby on board, it’s essential that you make healthy choices because what you eat, drink, and do may affect your baby. First things first, it’s time to amp up your nutrition game and make sure your diet includes plenty of lean protein, leafy greens, and enriched carbohydrates. If you haven’t already, start taking a daily prenatal vitamin that contains 600 to 800 mcg folic acid (also known as Vitamin B9), which is essential for lowering the risk of neural tube defects including spina bifida. 

If you’re a coffee or other caffeinated-beverage drinker, pay attention to the amount you’re consuming. Up to 200 milligrams a day of caffeine is believed to be a safe amount; there’s evidence that heavy caffeine intake in early pregnancy may slightly increase the chance of a miscarriage. And you know to say no to alcohol, marijuana, and recreational drugs when pregnant.

Exercise during pregnancy is also essential, though now is not the time to take up downhill skiing or start training for an ultra marathon if you weren’t a runner before! Stick with what you have been doing at the gym. If that isn’t much of anything, aim to add a few brisk walks or lap swims to your week. Keeping fit will make your pregnancy healthier, your labor and delivery faster, and your postpartum recovery easier.

Vitamin D is crucial to good health when you're four weeks pregnant and throughout your pregnancy. Fortunately, you can get it from being in the sunshine without sunscreen for at least fifteen minutes each day. Check with your medical practitioner if you’re reading this in the dark of winter; he or she might prescribe a supplement. Be sure to ask about the safety of any prescription drugs you take to make sure you should continue with those during pregnancy. Some prescription drugs may cause birth defects or other issues, so you’ll want an OK from your prescriber to continue taking them while pregnant.  

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