At 5 weeks pregnant, your baby is roughly the size of a sesame seed, measuring about 2 millimeters (less than one-tenth of an inch) from end to end. That’s tiny, but a remarkable amount of development is already underway inside that speck-sized embryo.
What You’d See on an Ultrasound
At this stage, you wouldn’t actually see anything that looks like a baby. A transvaginal ultrasound at 5 weeks typically shows a gestational sac, which is the fluid-filled structure surrounding the embryo, and possibly a yolk sac. The yolk sac measures about 3 to 5 millimeters across and serves as the embryo’s first source of nutrients before the placenta takes over. The embryo itself may or may not be visible yet, depending on the exact timing and the quality of the ultrasound equipment.
If your provider does an early ultrasound and doesn’t see much, that’s normal. Many practitioners prefer to wait until 7 or 8 weeks, when there’s more to measure and confirm.
What’s Happening Inside the Embryo
Size doesn’t tell the full story at 5 weeks. This is one of the most active periods of early development, when the embryo’s cells are rapidly organizing into three distinct layers that will eventually form every organ and tissue in the body.
The outer layer gives rise to the brain, spinal cord, nerves, and skin. The middle layer will become the heart, blood vessels, muscles, bones, and kidneys. The inner layer develops into the lungs, liver, and the lining of the digestive system. All three layers are actively differentiating at week 5, laying the groundwork for major organs even though none are functional yet.
The most significant milestone this week is the formation of the neural tube, which eventually becomes your baby’s brain and spinal cord. This is why folic acid is so important in early pregnancy: it supports the proper closure of this structure. The neural tube typically closes by the end of week 6, so this window is critical.
The Heart Starts to Beat
One of the earliest visible signs of life happens right around week 5. The developing heart is essentially two tiny tubes that have fused together in the center, branching into four smaller tubes. This primitive structure begins to contract, and by the end of the fifth week, it pulses roughly 110 times per minute.
Cardiac tissue contractions may be detectable on a transvaginal ultrasound between weeks 5 and 6, though this varies. It’s worth noting that what registers on the screen isn’t a fully formed heart beating. It’s cardiac tissue pulsing. The heart’s four chambers won’t develop until later. These early contractions are also not audible with the handheld Doppler devices used in prenatal office visits. Those typically pick up sound closer to weeks 10 through 12.
How 5 Weeks Is Counted
When your provider says you’re 5 weeks pregnant, they’re counting from the first day of your last menstrual period, not from the day of conception. Since ovulation and conception typically happen about two weeks into your cycle, the embryo is actually closer to 3 weeks old in terms of real developmental time. This can be confusing, but it’s the standard way pregnancy is dated because most people know when their last period started, while the exact day of conception is harder to pin down.
What’s Normal to Feel at 5 Weeks
Many people don’t feel pregnant at all this early. Others are already noticing symptoms driven by rapidly rising hormone levels. The hormone hCG, which pregnancy tests detect, ranges widely at 5 weeks: anywhere from 217 to 8,245 mIU/mL. That enormous range is normal and explains why symptoms vary so much from person to person.
Common experiences at this point include breast tenderness, mild cramping, fatigue, and the very beginning of nausea (though morning sickness more commonly ramps up around week 6 or 7). Some light spotting can also occur as the embryo implants more deeply into the uterine lining. The absence of symptoms doesn’t indicate a problem. Hormone levels and sensitivity to those hormones vary widely, and a symptom-free early pregnancy is perfectly normal.
How Size Changes From Here
Growth accelerates quickly after week 5. By week 6, the embryo roughly doubles in size to about 4 millimeters, and facial features like tiny pits where the eyes will form start to appear. By week 8, it reaches about half an inch long, and by the end of the first trimester at 12 weeks, it’s around 2.5 inches. The jump from a sesame seed to a lime happens in just seven weeks, which gives you a sense of how rapid this early growth phase is.
At 5 weeks, though, the most important developments aren’t about size. They’re about structure. The cells dividing right now are deciding what they’ll become, and the foundations for your baby’s nervous system, heart, and digestive tract are all being laid simultaneously. It’s a lot of work for something smaller than a grain of rice.