Era Health

Baby development at 3 weeks

Published 05 March 2025 | Written by Dr David Miller

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Baby development at 3 weeks

Cells are multiplying

Your developing baby is a tiny ball of several hundred cells that are multiplying and burrowing into the lining of your uterus. The cells in the middle will become the embryo. The cells on the outside will become the placenta, the pancake-shaped organ that delivers oxygen and nutrients to your baby and carries away waste.

Connecting to you

Your baby-to-be is receiving oxygen and nutrients (and discarding waste) through a primitive circulation system made up of microscopic tunnels that connect to the blood vessels in your uterine wall. The placenta will eventually take over this task around the end of the first trimester.

Pregnancy symptoms during week 3

No pregnancy symptoms?

Some women feel pregnant even before the test is positive, but most don't. If you have pregnancy symptoms this week, they may feel like PMS. But don't worry if you haven't felt anything different yet. Even at 5 weeks pregnant, only half of women feel pregnancy symptoms.

Gas and bloating

The hormone progesterone relaxes muscles throughout your body, including in your digestive tract. These relaxed muscles slow down your digestion, which can lead to gas and bloating and create uncomfortable sensations in your gut. About half of pregnant women experience constipation at some point during their pregnancy. To keep things moving, stay hydrated and eat high-fiber foods like whole grains, fruits, and vegetables.

Sore breasts

Some women say their sore breasts in early pregnancy are like an exaggerated version of how they feel before a period. Your breasts may feel swollen, tender, or tingly – and your nipples may be extra sensitive and uncomfortable. Later in pregnancy, you may notice your nipples getting darker.

Spotting

You may have a bit of light bleeding (spotting) this week. This is implantation bleeding – it can happen around the time the fertilized egg implants in your uterus. It's a lot lighter than a typical period, and only lasts for one to three days. (If you have pain along with bleeding, call your healthcare provider immediately. This can be a sign of an ectopic pregnancy.)

Basal body temperature stays high

If you're charting your temperature, it should stay elevated this week. To keep track, use a basal body thermometer and take your temperature after you wake up in the morning, before you get out of bed.

Pregnancy checklist at 3 weeks pregnant

Pay attention to your emotions

When you're waiting to learn whether you're pregnant or not, or just finding out, it's normal to be anxious. If you're feeling stressed or worried, talk to your partner or a trusted friend. Or, try writing down everything that's bothering you. Journaling can improve your emotional well-being, mental clarity, and even physical health.

Avoid overheating

Hot baths are okay during pregnancy as long as they aren't too hot. But avoid steam baths, hot tubs, and saunas. Elevated body temperature, especially early in pregnancy, has been associated with an increased risk of neural tube defects in babies.

Eat nutritious meals and snacks

Eat pregnancy-friendly foods such as fruits and vegetables, low-mercury fish, and whole grains. Choose foods containing vitamin C (like strawberries, citrus fruits, bell peppers, and tomatoes), iron (like beef, poultry, soy products, and spinach), and calcium (like Greek yogurt, fortified cereal, and pasteurized cheese). For snack ideas, check out 10 of our favorite healthy snacks for expecting moms.

Cut down on coffee

While you're trying to conceive and once you get pregnant, experts recommend limiting caffeine to about one cup of coffee a day. It's important to watch your caffeine intake because having too much can affect your pregnancy and your baby. See how much caffeine is in different foods and drinks so you can stay under the recommended daily limit.

Get help quitting

If you need help quitting smoking, drinking, or taking drugs, talk to your healthcare provider and ask for a referral to a program or counselor.

Improve your sleep

When pregnancy-related sleep problems hit in a few months, be ready for them. Create better habits around sleep and work on good sleep practices like establishing a regular bedtime routine and making your bedroom a sleep sanctuary.

Make sure your work and home are safe

Some jobs can be hazardous to you and your baby. If you're routinely exposed to chemicals, loud noises, or radiation at work, or if you have to be on your feet constantly or lift heavy objects, talk to your provider about how to stay safe while working during pregnancy. Also, keep in mind that some things in your home may be dangerous for your developing baby. Lead (in drinking water from old pipes), mercury (in certain fish), certain pesticides and fertilizers, and cat litter containing feces are all potentially harmful.

Kate Marple is a writer and editor who specializes in health, pregnancy, and parenting content. She's passionate about translating complicated medical information into helpful pregnancy and parenting advice that's easy to understand. She lives in San Francisco with her family.