Early Pregnancy Fetal Development


Early Pregnancy

Fetal Development as Seen Via Ultrasound
Photo © E+/Getty Images

The egg and sperm meet in the ampulla, the second segment of the fallopian tube. Once the egg is fertilized and a zygote is formed, a sequence of cell divisions follows. From here the embryo will reach the blastocyst stage and implant in the uterine lining. The inner cells of the embryo become the fetus. The outer cells become the amniotic sac, which surrounds the fetus through the duration of the pregnancy, and the placenta, which is attached to the uterine wall, begins to grow and nourish your baby.

Loading shell for quizzesApp1 vue props component in Globe.

4 Weeks of Pregnancy

Pregnancy Week 4
Photo © Science Picture Co/Getty Images

Prenatal care is so important to a baby's health—even if you don't know you're pregnant yet. As your body prepares for a growing baby, you may show some signs of early pregnancy at four weeks like bloating, breast tenderness, mood swings and nausea, and it's right around now that you could find out that you're pregnant.

Meanwhile your embryo will go through three stages of fetal kidney development: pronephros, mesonephros and metanephros. During the fourth week, the first stage (pronephros) disappears so that the second stage (mesonephros) can begin forming.


7 Week Embryo

Pregnancy Week 7
Photo © Science Picture Co/Getty Images

The hand plates become present this week, and the baby is about 1.5 centimeters crown-rump length (CRL) by the end of the week (at the start of the eighth week). The genital tubercle is present, but you can't distinguish the sex by sight at this point. Nasal pits are also forming.

Bones and joints this week have many things going on. The process of ossification (hardening of the bones) begins.

With a transvaginal ultrasound, one study shows that over 90 percent of ultrasounds will show a fetal pole with heart motion.


8 Week Fetus

Pregnancy Week 8
Photo © Science Picture Co/Getty Images

The embryo now enters the fetal period. Your baby will be about 2.2 centimeters long at the end of the eighth week (and at the start of the ninth). By now, the fetal brain weighs an impressive 43 percent of the total mass of the fetus. This week the baby's gonads will become either testes or ovaries. The baby's fingers and toes are webbed and spontaneous movement begins! There are a lot of things that we do know about life before birth.

All major organs have begun to form by the end of the eighth week.


10 Week Fetus

Pregnancy Week 10
Photo © Science Picture Co/Getty Images

At the start of the tenth week, the average size of the fetus is approximately 3.2 centimeters CRL and weighs about four grams. Tiny toes formed in the ninth week, and now toenails and fingernails are growing. The eyelids have fused shut and will stay that way until they separate at around 28 weeks. External ears are continuing to develop, as well as the upper lip. External genitalia is beginning to differentiate.

The fingers and toes are no longer webbed and now the baby is kicking around!

The embryonic tail disappeared during the ninth week.


12 Week Fetus

Pregnancy Week 12
Photo © Science Picture Co/Getty Images

While your baby's brain is not quite yet the size it will be at birth, it has begun developing its structures. Your fetus weighs about 14 grams and is approximately 5.5 centimeters long.

Your baby has developed suckling reflexes and their kidneys are producing urine. All of this in preparation for extrauterine life.

If your practitioner uses a doppler, there's an 80% chance you may be able to hear your baby's heartbeat at this office visit. It is a joyous sound to hear!

Your chance of miscarriage is greatly reduced after 12 weeks gestation.


16 Week Fetus

Fetal Development as Seen Via Ultrasound
Photo © E+/Getty Images

The ears and eyes are close to reaching their final positions.

The nails are well on their way to reaching their full length, which happens at around 17 weeks. Following birth, babies' nails grow very fast and need to be trimmed regularly.

Urine production has increased significantly. The limb movements are becoming more coordinated. At 16 weeks, Your baby is 11.5 centimeters CRL and weighs about 100 grams).

The sex of your baby may be detectable by ultrasound.


24 Week Fetus

Pregnancy Week 25
Photo © Science Picture Co

By now your baby has been surrounded by noise for a long time and is starting to hear you and those around you. Things like your heartbeat, digestion, and other body functions are heard by the baby as well as external noises, since the nerves in their ears have begun to develop.

Your baby's brain is rapidly growing and they are also developing their taste buds. They weigh in at 600 grams (1.3 pounds) and are about 30 centimeters (12 inches) crown-heel length (CHL).

They are also beginning to deposit brown fat (adipose tissue) on their body around mid-gestation. The purpose of the brown fat is to retain body heat. Newborns may struggle with regulating body temperature at first. This is particularly a problem for a baby born early, since they have not developed enough fat on their body.

Babies born at this point have some chances of survival with very special care. They will be in the Neonatal Intensive Care Unit (NICU), probably for many weeks. In most cases, you can expect them to stay in the NICU until close to their due date.

A major problem with premature babies is lung development. If preterm labor is detected early, a steroid shot called Betamethasone can sometimes be given to enhancing the lung development.


26-28 Week Fetus

Pregnancy Week 27
Photo © Science Picture Co/Getty Images

At 26 to 28 weeks, bile has begun developing the function of secretion, a process that is not completed until 2 to 5 weeks following birth.

Your baby's brain is also becoming more complex and has developed a regular sleep-wake cycle. Although it usually seems as if the baby wants to sleep while you are awake and vice versa at night.

At 27 weeks your baby is 36.5 centimeters (14.4 inches) long and weighs in at about 875 grams (just under 2 pounds) CHL.


30-39 Week Fetus

Pregnancy Week 31
Photo © Science Picture Co/Getty Images

At 30 weeks there's only about 10 left to go until you meet your baby!

At 32 weeks, your baby now weighs 1.7 kilograms (3.7 pounds) and is approximately 42 centimeters (16.5 inches) long CHL.

At 33 weeks, your baby's skeleton is fully formed and the bones are beginning to harden. Your baby's eyes have matured enough that they're able to notice the difference between light and dark.

For the next few weeks your baby will continue to put on weight. At 39 weeks, a layer of fat is continuing to develop under their skin to keep your baby warm following birth.

If you are having Braxton Hicks Contractions use them to practice what you've learned in childbirth classes, and know that they are a sign that your body is getting ready for labor. The baby notices the contractions but is not adversely affected by them.

Was this page helpful?
25 Sources
Verywell Family uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Georgadaki K, Khoury N, Spandidos DA, Zoumpourlis V. The molecular basis of fertilization (Review). Int J Mol Med. 2016;38(4):979-86. doi:10.3892/ijmm.2016.2723

  2. Kim SM, Kim JS. A Review of Mechanisms of Implantation. Dev Reprod. 2017;21(4):351-359. doi:10.12717/DR.2017.21.4.351

  3. Stanford Children's Health. Nutrition Before Pregnancy.

  4. Eunice Kennedy Shriver National Institute of Child Health and Human Development. What are some common signs of pregnancy?.

  5. LifeMap Discovery - Embryonic Development & Stem Cell Compendium. Stages in Kidney Development: Pronephros, Mesonephros, and Metanephros

  6. Papageorghiou et al. University of Oxford. International Standards for Fetal Crown Rump Length by Gestational Age. Lancet. 2014;384:869-79.

  7. Rankin J. Physiology in Childbearing E-Book: With Anatomy and Related Biosciences. Fourth Edition. Elsevier: Amsterdam; 2017: 98.

  8. Wiśniewski M, Baumgart M, Grzonkowska M, Szpinda M, Pawlak-osińska K. Quantitative anatomy of the ulna's shaft primary ossification center in the human fetus. Surg Radiol Anat. 2019;41(4):431-439. doi:10.1007/s00276-018-2121-2

  9. Makiyan Z. Studies of gonadal sex differentiation. Organogenesis. 2016;12(1):42-51. doi:10.1080/15476278.2016.1145318

  10. The American College of Obstetricians and Gynecologists. How Your Fetus Grows During Pregnancy.

  11. Bhattacharjee D. FirstCry Parenting. Foetal Growth Chart Week by Week — Length & Weight.

  12. Tawfik HA, Abdulhafez MH, Fouad YA, Dutton JJ. Embryologic and Fetal Development of the Human Eyelid. Ophthalmic Plast Reconstr Surg. 2016;32(6):407-414. doi:10.1097/IOP.0000000000000702

  13. Blaschko SD, Cunha GR, Baskin LS. Molecular mechanisms of external genitalia development. Differentiation. 2012;84(3):261-8. doi:10.1016/j.diff.2012.06.003

  14. Zhang ZH, Lin XT, Teng GJ, et al. Mapping fetal brain development in the second trimester with 7.0T magnetic resonance imaging. Zhonghua Yi Xue Za Zhi. 2013;93(3):204-7.

  15. Rowland J, Heazell A, Melvin C, Hill S. Auscultation of the fetal heart in early pregnancy. Arch Gynecol Obstet. 2011;283 Suppl 1:9-11. doi:10.1007/s00404-010-1563-5

  16. Mayo Clinic. Pregnancy week by week. Fetal development: The 2nd trimester.

  17. Hindryckx A, De catte L. Prenatal diagnosis of congenital renal and urinary tract malformations. Facts Views Vis Obgyn. 2011;3(3):165-74.

  18. Kearin M, Pollard K, Garbett I. Accuracy of sonographic fetal gender determination: predictions made by sonographers during routine obstetric ultrasound scans. Australas J Ultrasound Med. 2014;17(3):125-130. doi:10.1002/j.2205-0140.2014.tb00028.x

  19. Symonds ME, Pope M, Sharkey D, Budge H. Adipose tissue and fetal programming. Diabetologia. 2012;55(6):1597-606. doi:10.1007/s00125-012-2505-5

  20. Stanford Children's Health. Keeping Your Baby Warm.

  21. Gaur K, Ganguly B. Effect of Single Dose Betamethasone Administration in Pregnancy on Maternal and Newborn Parameters. J Clin Diagn Res. 2017;11(5):FC15-FC18. doi:10.7860/JCDR/2017/25459.9819

  22. Lester R. Bile acid metabolism in the fetus and newbornCiba Found Symp. 1979;(70):99-115. doi:10.1002/9780470720530.ch6

  23. Okawa H, Morokuma S, Maehara K, et al. Eye movement activity in normal human fetuses between 24 and 39 weeks of gestation. PLoS One. 2017;12(7):e0178722. doi:10.1371/journal.pone.0178722

  24. Mayo Clinic. Pregnancy week by week. Fetal development: The 3rd trimester.

  25. The American College of Obstetricians and Gynecologists. How to Tell When Labor Begins.