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Baby Development By Trimester

From when you can hear your baby’s heartbeat to what her first movements feel like, here’s how your baby develops in each trimester.

by: Sydney Loney

ondering what your baby is up to in your belly? We asked world-renowned radiologists, Drs. Peter Doubilet and Carol Benson, the five most common questions moms-to-be have when it comes to fetal development.

1. When will I be able to hear my baby's heartbeat?

At around 10 weeks, a Doppler ultrasound will allow you to hear your baby's heartbeat for the first time–but her heartbeat will already be visible on ultrasound by the time you are about six weeks pregnant. (To see what your baby's heartbeat looks like at six weeks, check out Drs. Doubilet and Benson's site: yourdevelopingbaby.com.)

"The first detection of the baby's heartbeat has both medical and emotional significance," says Dr. Doubilet. "From a medical standpoint, demonstration of the heartbeat is the first definitive proof that the pregnancy contains a live baby. For the parents, the first time they see or hear the heartbeat is a joyous and exciting moment as they first witness the pulse of a new life inside the mother."

When you hear your baby's heartbeat for the first time, don't be alarmed if it sounds fast. At six weeks, your baby's heart beats at about 100 to 120 beats per minute, speeding up to about 160 to 180 beats per minute at 10 weeks, then slowing to 130 to160 beats per minute by the beginning of the second trimester, where it will remain for the rest of your pregnancy.

2. When can I find out my baby's gender?

Your mid-pregnancy ultrasound (usually done between 16 and 20 weeks) involves a detailed anatomical analysis of your baby and is usually when you will be able to find out if you're carrying a boy or a girl–if you want to, that is.

"In our experience, about 80 to 85 percent of parents want to know the sex of their baby when they come for an ultrasound," say Dr. Benson. "If we get a good look between the legs, as we can in most pregnancies beyond 15 to 16 weeks, ultrasound is close to 100 percent accurate at sex determination."

However, the ability to determine the baby's sex depends on a number of factors, including the position of the baby, whether his/her legs are open and whether his/her hands are covering the "private parts." The mother's weight is another factor. "Ultrasound images are far less clear in heavy women than they are in thinner women, so we may not be able to figure out the baby's sex in a heavy woman until 18 to 20 weeks."

3. When can a baby hear sound from outside the womb?

Some studies have suggested that babies begin hearing at 20 weeks of pregnancy, and that they are able to distinguish their mother's voice, says Dr. Doubilet.

"What we know with certainty, based on direct ultrasound visualization, is that the baby responds to loud noises or buzzing sounds in the third trimester. When we do an ultrasound while placing a small buzzer on the mother's abdomen, we can watch the baby make a sudden movement of his/her arms and legs in response to the buzzes. This startle reaction is a reassuring sign of good health."

4. When will I be able to feel my baby move?

Although your baby probably began moving around eight weeks, if this is your first pregnancy you may not feel her move until you reach about 18 to 22 weeks, or longer. (Some women describe the sensation as a "gentle fluttering" that feels like bubbles, butterflies or gas.)

If this is your second or third pregnancy, you might feel movement earlier, at about 16 to 18 weeks. This may be because you've been through pregnancy before and can recognize the subtle movements your baby makes earlier. Around the 24th week, you may feel a rhythmic, jerking sensation if your baby gets the hiccups.

"In the second and third trimesters, parents can get their first sense of their baby's personality as they watch it not only move its arms and legs, but also open and close its mouth, turn its head from side to side, and even occasionally suck its fingers or wink," say Dr. Benson. Click here to see videos of babies moving on their site, Your Developing Baby.

How often your baby moves varies from woman to woman, baby to baby and even time of day, but you should probably expect to feel her move several times a day. If you're worried, try lying down, resting and focusing on your baby at a time of day when she's normally active. If it's been a day or more since you last felt movement, contact your caregiver just to help put your mind at ease.

5. What makes each trimester unique when it comes to fetal development?

First trimester: "The first trimester is the most amazing and spectacular three-month period in human biology," say Dr. Doubilet. Starting from a single cell (the fertilized egg) all of the baby's internal organs will be in place by the end of the first trimester. "It is a time of incredibly rapid growth, with the baby increasing in length from a tenth of an inch at six weeks to three inches at 13 weeks (the end of the first trimester)."

Click here for a detailed look at your baby's first trimester, including 3-D ultrasound pictures.

Second trimester: During the second trimester, your baby begins to look like a newborn as she develops recognizable human features and her body becomes more proportionate. She may be able to make facial expressions, start to swallow and suck her thumb. "By looking closely at the baby's external features and internal organs, ultrasound allows the doctor or other caregiver to make sure that he/she is developing normally," says Dr. Benson.

Click here for a detailed look at your baby's second trimester, including 3-D ultrasound pictures.

Third trimester: The third trimester is when the baby develops into a being capable of surviving on her own as her lungs, brain and other internal organs mature. Her bones are fully developed and she is now able to open and close her eyes and can detect light. (She is considered full-term at 37 weeks). During this trimester, your baby will also gain a lot of weight, going from an average of 1½ pounds at 25 weeks to about 7½ pounds at birth.

Click here for a detailed look at your baby's third trimester, including 3-D ultrasound pictures.

Meet our experts:

Dr. Peter Doubilet and Dr. Carol B. Benson are Harvard Medical School professors of Radiology. They teach and see patients at the Brigham and Women's Hospital in Boston where Peter is Senior Vice Chair of Radiology and Carol is Director of Ultrasound and Co-Director of High Risk Obstetrical Ultrasound. They are co-authors of Your Developing Baby (McGraw-Hill, 2008), a pregnancy guidebook that takes readers through every stage of a baby's development, using 2D and 3D ultrasound images, original explanatory diagrams and reader-friendly prose.