Twin Fetal Development Overview

by Brian M. Williams

Twins Early UltrasoundCongratulations, you're having twins! Here's what you need to know about your twins fetal development.

Most moms-to-be get two or three ultrasounds. You'll most likely be offered enough scans for a virtual slide show! Your doctor monitors the babies' growth individually and compared to their sibling.

Your unborn babies will grow at the same rate as a singleton until about weeks 28-30, but times two! From then on, twins tend to lag slightly behind in growth. Some researchers say that there is a possibility of less nutrients contributing to a slower growth rate. Other scientists suggest that crowding in the womb might slow things down.

Factors Affecting Twin Fetal Growth

Type of twin: Twins who have their own placenta tend to grow faster in the latter part of pregnancy.

Gender: Boy-boy twins have lower growth rates and birth rates when compared to boy-girl and girl-girl twins.

Mom's health: Conditions like diabetes and high blood pressure affect your babies' growth. Smoking and even your age can affect fetal growth.

Discordant Fetal Growth

During a pregnancy with twins or higher order levels of multiples, it's normal for the babies to be slightly different sizes. If the difference in size gets too large, your provider might start to get concerned.

Once the size difference reaches 15 to 25 percent, the babies are said to have "discordant fetal growth." The growth difference can have a number of causes including genetic, intra-uterine growth syndrome and twin-to-twin-transfusion. Your doctor will watch the babies closely and could suggest an early induction or cesarean.

Feeding Your Fetuses

Some studies suggest that when the condition in your womb meets your babies needs, their birth weight increases and differences in weight decrease. While good nutrition isn't the only reason a baby grows well, plenty of nutrients and other building blocks help. To make sure your babies get their fair share, keep these key points in mind.

You get to gain more weight. Moms-to-be of twins (or higher) are allowed to gain more weight. Those extra pounds grant you the license to eat about 600 additional calories per day. Spend those extra calories on nutrient rich foods that build healthy bodies.

You'll increase certain nutrients. You'll need more folic acid, calcium, iron and protein to meet the needs of two growing babies. If you're already eating a healthy diet, keep it up. If you're not certain, ask to meet with a dietician to grant you peace of mind.

You'll have more visits to your healthcare provider. Since you'll have more than a mom who is carrying just one baby, it's important that you don't miss your appointments. Your healthcare team can use your weight and babies' growth data to suggest changes in your menu or daily routine.

Coming Soon: Growth Standards for Twins

The National Standard for Normal Fetal Growth Study, sponsored by the National Institutes of Health, started in 2009 and runs through March 2012. The research uses ultrasound to redefine what represents normal fetal growth in healthy women pregnant with one child.

Its follow up, The National Twins Study, is recruiting healthy women expecting twins. The data collected will allow investigators to develop customized fetal growth curves that can be adjusted for various maternal factors and for fetal gender.

"These studies will set new standards for normal fetal growth using ultrasounds instead of the previous standard, which involved measuring the fundal height," stated Roger Newman, M.D., the study’s principal investigator.

Dr. Newman says that the twin study will answer questions about the differences between single and twin gestations, between identical and fraternal twins and what degree of growth discordance between co-twins is clinically acceptable. The new standards can help doctors know when they should or should not intervene in a pregnancy.

Are you carrying twins or more? How's your pregnancy going? Tell us all about it!