Understanding Fetal Development Milestones (Week by Week)
Tracking the milestones of fetal development can feel like a remarkable journey. From conception to birth, your baby goes through incredible physical transformations—all in the span of 40 weeks. Get an in-depth look at when and how these developments occur by exploring the key stages of fetal development week by week.
First things first: Let’s talk about gestational weeks
When we’re talking about pregnancy milestones, we use gestational weeks. This means we consider your pregnancy's start date as the first day of your last menstrual period – even though you weren’t technically pregnant yet in the sperm-meets-egg sense.
Referring to the age of pregnancy in gestational weeks (also called “gestational age”) is a common practice because it’s usually easier to pinpoint the start of your last period than to figure out the exact day your egg was fertilized and became a “zygote.” Even if you’re blessed with regular 28-day cycles, there’s no guarantee that you ovulate on the same day each month. What’s more, there’s no way to know exactly when the zygote, rapidly replicating its cells in the process of becoming a “blastocyst,” traveled down the fallopian tube and implanted into the uterus.
First Trimester Pregnancy Milestones
Major first-trimester pregnancy milestones include:
Week 1: Conception and Implantation
Week 2(ish): Fertilization! Sperm + egg = zygote, complete with genetic information from each parent.
Week 3: Around a week after conception, the zygote will have grown to have many more cells, and is now considered a blastocyst. Successful intrauterine pregnancies occur when a blastocyst first implants within the uterine wall and starts to grow.
Week 4: As early as a week before your first missed period, you may get that “Big Fat Positive” on your pregnancy test!
Week 6: Your OB care provider may perform a “viability” ultrasound in the office, or refer you to an ultrasound center for this exam, which will confirm that the gestational sac is found inside your uterus. This will rule out the possibility of an ectopic pregnancy - that is, when the blastocyst implants outside of the uterus, such as in the fallopian tube.
There’s lots happening in fetal development during week 6, including the growth of the early “heart tube.” You may be able to hear a heartbeat on a sonogram.
You won’t likely be able to see many features of your baby at this point, but it can still be an exciting moment to see your pregnancy on the screen for the first time!
Week 7: Facial features start to develop, and the embryo grows tiny buds that will eventually become arms and legs. Even though the chromosomal sex of the baby (XX or XY) is determined at the moment of fertilization, it’s not until the beginning of 7 weeks gestation that the very early fetal genital system starts to develop. Prior to this point, embryos have “indifferent gonads” which appear exactly the same in both XX and XY embryos! For parents who don’t want to wait for an ultrasound to tell them the sex of their baby, week 7 is the earliest point to use an at-home early gender test to find out the sex of your baby (check out our Birch™ Fetal Sex Test for more information).
Week 8: The baby’s intestines and liver begin to grow very rapidly, and in all pregnancies, will become too large for the size of the abdomen. These organs actually exit the abdominal cavity through the umbilical opening, where they will gradually rotate within the umbilical cord for the next 3-4 weeks as the abdomen grows in size. This process, called abdominal herniation, is how the complex folding of the small and large intestines develops within all babies!
Week 11: Your OB provider may recommend a special ultrasound called a “nuchal translucency” (NT) sonogram, also called a “first-trimester screening” sonogram, between the beginning of week 11 and the end of week 13. This sonogram carefully examines the fluid-filled space at the back of the fetal neck (“nuchal translucency”) to compare this measurement with the expected range for the baby’s size. Although the fetal heart is very small and hard to see by sonogram in the first trimester at most doctors’ offices, an increased nuchal translucency space can indicate atypical fetal heart development. Since about 1 in every 100 babies has a difference in their heart structure (also called a “heart defect”), this screening is important to help your provider recommend the best next steps for monitoring your baby’s health.
The NT sonogram at 12-13 weeks will also usually reveal that the intestines are back inside of the abdomen and no longer inside of the umbilical cord since the process of intestinal herniation is almost always completed by 12 weeks.
Week 14: The fetus has fully-formed arms, hands, fingers, feet, and toes - now they just need to keep growing bigger! Ultrasounds at this point might even catch the fetus starting to explore its body and surroundings by opening and closing its fists and mouth.
In the first trimester, amniotic fluid (the liquid surrounding the fetus) is mostly made from maternal serum. However, in the second trimester, fetal urine production begins to quickly increase so that, eventually, all of the amniotic fluid is fetal urine! As the fetus grows in the amniotic fluid, it will consistently make and swallow this fluid, helping to gradually expand the lungs - similar to a pop-up sponge!
Because there are so many developmental milestones in the first trimester, reaching the end of month three is cause for celebration! After this point, your risk of miscarriage drops significantly, and many consider this to be a reasonable time to start telling your friends and family you’re expecting if you haven’t already.
Second Trimester Pregnancy Milestones
Fetal developmental milestones during the second trimester include:
Weeks 15–18: Say goodbye to your morning sickness! Also, by this point, the fetus can turn over in the womb, make faces, yawn, and suck its thumb.
Although both hemispheres of the fetal brain developed early in the first trimester, the most enlargement of the brain occurs after about 15 weeks of gestation.
Weeks 19–22: You’ll probably begin to feel the fetus moving around, a process referred to as “quickening.” You’ll also likely have your anatomy ultrasound around this time, and your technician may be able to tell you the baby’s genetic sex based on the appearance of the genitalia.
Weeks 23–26: By now, other people should be able to feel the baby’s movements by touching your belly (as long as they ask first)! The baby may also respond to sounds, and you may notice tiny jerking movements when they get hiccups.
Third Trimester Pregnancy Milestones
Each week of your third trimester, your baby reaches important developmental milestones that will prepare them for birth. This is when they’ll gain the bulk of their weight (aka “baby fat”). Here’s what else your baby is up to during your final months of pregnancy:
Weeks 27–30: The baby’s eyelids open, and they can react to light. Their lungs are now developed enough that, if born prematurely, intensive care would likely help them to survive.
By week 26, the baby’s brain has grown considerably, and the grooves and ridges of the brain (called “sulci” and “gyri”) become much more distinct.
Week 35: By now, the fetus has developed some major reflexes! They can blink, close their eyes, clench their fists, and react to light, sounds, and touch.
Week 37: Congratulations, you are now officially full term! During your final month of pregnancy, you could go into labor at any time.
Week 39: You’ll notice things getting pretty tight in your womb, and it may get a little uncomfortable as the baby drops to prepare for birth.
At this point, all that’s left is to deliver your baby! The past nine to ten months have all been building up to this moment, and the next step is to (finally) bring your new baby home. In the weeks and months after your delivery, the main focus will be on your little one, but try not to forget to take care of yourself too!
Our patient, Kelsey Speelman shares her pregnancy journey with you on Instagram:
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Please note: JunoDx.com and the materials and information it contains are not intended to, and do not constitute medical or other health advice or diagnosis and should not be used as such. You should always consult with a qualified physician or health professional about your specific circumstances.