Your Baby’s Development in the First Trimester Explained for Dads

By Mathew Miller, Sharon Perkins

When the embryo first implants in the uterus, about a week before a menstrual period is missed, it’s too small to be seen without a microscope. Within a week, though, you can see the first signs of pregnancy via vaginal ultrasound.

Although the embryo still isn’t discernible, the gestational sac that surrounds the embryo shows up as a small black dot. From this point on, fetal growth is an astounding miracle.

He may not look like much now, but…

In six weeks, your baby embryo grows from a ball of cells to a recognizable creature, although the exact species is difficult to define. Following are the changes that occur in the first six weeks of pregnancy, which include the first four weeks, the time from the last menstrual period to the first missed period.

  • Week 2: Egg and sperm meet, usually in the middle of the fallopian tube. The zygote formed by the union of egg and sperm drifts down to the uterus over several days.

  • Week 3: Implantation occurs 7 to 12 days after fertilization. There may be a small amount of implantation bleeding as the embryo burrows into the uterine lining.

  • Week 4: Your partner misses her menstrual period. A pregnancy test, which detects minute amounts of human chorionic gonadotropin, or hCG, may be positive as early as week 4. The embryonic cells divide into two sections during this week, one that will become the embryo and one that will become the placenta.

  • Week 5: The yolk sac, which nourishes the embryo before the placenta forms, may be visible next to the gestational sac on ultrasound. The embryo now consists of three layers that will develop into different areas of the body. On ultrasound, a small dark spot, the gestational sac, may be seen.

  • Week 6: During this week, the embryo looks like a bent-over bean with a slight curve at the end. The heart is still a primitive tube, but you can see a flickering heartbeat on ultrasound as blood begins to circulate. Arm and leg buds are sprouting, and the eyes, ears, and mouth begin to form, although they’re still a long way from a finished product at this point.

Incredible changes in weeks 7 to 12 explained for dads

Although few people would say “Yes, sir, that’s my baby” by week 6, between weeks 7 and 12, the embryo really starts to look human.

  • Week 7: In week 7, the baby is huge — around the size of a blueberry! At least he’s something you can see with your own two eyes, and it’s a 10,000-times increase over his original size. The brain and the internal organs are all growing, and the arms and legs have primitive hands and feet.

  • Week 8: Fingers and toes start to form, and the nervous system starts to branch out. Those new limbs are moving, although it’ll be weeks before your partner can feel movement, even if she swears she’s feeling it already.

  • Week 9: The baby’s heartbeat may be audible using a Doppler, which amplifies sound. You’ll never forget the first time you hear that rapid beat and realize that a real human is attached to it.

  • Week 10: The kid doesn’t even have knees yet, and he’s already forming teeth in his gums! He does have elbows, though, and knees aren’t far behind.

  • Week 11: Your 2-inch bundle of joy is beginning to look like a real miniature person, one who has brand-new fingernails and an admittedly large head.

  • Week 12: The internal organs are growing so much that they protrude into the umbilical cord (they’ll start moving back into the abdominal cavity shortly), and the baby is making urine.

    [Credit: Illustration by Kathryn Born, MA]

    Credit: Illustration by Kathryn Born, MA