With the overview out of the way, we can dive right into the specifics. Keep reading for what to know about expected pregnancy progress—including body changes and fetal development by trimester—so you’re up to speed on the major milestones.
Fetal Stage 1: First Trimester (Week 1-Week 12)
The first trimester can be a complex period wrapped up in excitement, worry, body changes, and “morning” sickness (it’s really more of an all-day thing)—among so many other feelings and emotions. And for at least one-third of the trimester, many people won’t even know they’re pregnant!
It’s for all of those reasons that knowing what to expect, both internally and externally, can be really helpful. The TL;DR? The first trimester is all about fertilization and organ development.
How Your Body May Change
According to the American College of Obstetricians and Gynecologists (ACOG), which is the leading national organization of OB/GYNs, these are some of the possible body changes to expect in the first trimester (2):
• No more periods
• Larger and more tender breasts/chest
• Nipples that stick out more
• More frequent urination
• Tiredness
• Nausea and vomiting
• Food cravings or loss of appetite
• Heartburn or indigestion
• Bloating and gas
• Constipation
• A few pounds of weight gain or loss
How the Pregnancy Develops
Let’s begin with what happens in the weeks leading up to a pregnancy:
• About two weeks before a pregnancy test can show a positive result, an
ovary releases an egg so it can be fertilized by sperm. (2)
• Sperm in the reproductive tract fertilize the egg within 24 hours of ovulation (that’s how long the egg can live). (3) The fertilized egg travels through the fallopian tube to the uterus.
• 1-2 weeks after ovulation, the fertilized egg implants itself into the lining of the uterus. (4) The cells surrounding the implanted now-embryo begin producing human chorionic gonadotropin (hCG). These cells eventually form into the placenta.
• Home pregnancy tests are most accurate at detecting hCG in the urine 1-2 weeks after a missed menstrual period. (5) However, some tests may detect hCG as soon as ~12-15 days post-ovulation. (6)
ACOG breaks down embryo and fetal development by week. Here’s what you need to know about embryo development in weeks 1-8:
• The embryo’s brain and spine (neural tube) begin to form.
• The embryo’s cardiac tissue, which will become the heart, starts to develop.
• The embryo’s eyes, nose, and mouth muscles form.
• The embryo’s developing hands and feet begin to show webbed fingers and toes.
• The embryo’s urinary system begins to develop. (10)
• The embryo’s lungs begin forming the tubes that will eventually carry air in and out.
• The embryo’s inner ear starts to develop.
The formation of the baby’s neural tube is why prenatal multivitamins with folate are recommended at least one month before getting pregnant. (11) Choline supports baby's neural tube development during pregnancy.* The fetal neural tube begins to form during weeks 3 and 4—before most people even realize they’re pregnant. (12)
What you need to know about fetal development in weeks 9-12:
• In the fetal stage as of week 10, the fetus's limbs, hands, and feet cartilage begin to form.
• The fetus's eyelids form but stay closed.
• The fetus's genitals and liver begin to develop.
• The fetus's kidneys and pancreas start producing urine and insulin.
• The fetus's fingernails form.
