First Trimester Week by Week: What to Expect
A warm, detailed guide to weeks 1 through 13 what's happening with your baby and your body each week.
The first trimester is simultaneously the most invisible and the most transformative period of your entire pregnancy. From the outside, nothing looks different. But inside your body, an absolutely extraordinary process is underway. Here's what's really happening week by week during these remarkable first 13 weeks.
Weeks 1, 2: The Very Beginning ๐ฑ
Interestingly, during weeks 1 and 2 you aren't actually pregnant yet, at least not in the conventional sense. Week 1 starts on the first day of your last menstrual period, and conception typically occurs around day 14 of your cycle during ovulation. This is why pregnancy is counted from your last period rather than from the moment of conception.
Week 3: Conception! โจ
This is when the magic actually happens. A single sperm fertilizes your egg in the fallopian tube, creating a zygote containing 46 chromosomes, 23 from you and 23 from your partner. This microscopic bundle of cells already contains everything that will determine your baby's eye color, hair color, height, and countless other traits. It begins traveling down toward the uterus, dividing and growing with every hour.
Week 4: Implantation ๐
The rapidly growing ball of cells, now called a blastocyst, burrows into the lining of your uterus in a process called implantation. This is when you might notice very light spotting. Your body begins producing hCG, the pregnancy hormone, and a home pregnancy test will now show a positive result. Your baby is currently about the size of a poppy seed.
Weeks 5, 6: The Heart Begins Beating ๐
By week 5, the neural tube that will become your baby's brain and spinal cord is forming. By week 6, something truly miraculous happens, your baby's tiny heart begins beating. It's incredibly small, but it's beating. This is also when morning sickness typically begins for many women, as hCG levels rise rapidly. Your baby is now the size of a sweet pea.
Weeks 7, 8: Features Form ๐ถ
Your baby is growing at a breathtaking pace, doubling in size almost weekly. Tiny arm and leg buds are forming and growing longer. The face is beginning to take shape, with dark spots where the eyes will be, and small indentations where the ears and nose will form. All the major internal organs are beginning to develop. By week 8, your baby officially graduates from embryo to fetus, a real milestone.
Weeks 9, 10: Fingers and Toes ๐๏ธ
This is one of the most exciting developmental periods. Your baby's tiny fingers and toes are forming, separating from what were previously paddle-shaped hands and feet. The teeth are beginning to form inside the gums. Your baby can now make tiny spontaneous movements, though you won't feel them for many more weeks yet. The face is becoming increasingly recognizable as a human face.
Weeks 11, 13: End of the First Trimester ๐
You are almost through the first trimester! By week 11, all of your baby's major organ systems have formed and are beginning to function. By week 12, your baby can yawn, make facial expressions, and even suck their thumb. The risk of miscarriage drops significantly around this time, which is why many couples choose to share their pregnancy news at the end of the first trimester. By week 13, your baby is about the size of a lemon and weighs nearly 23 grams.