1. How Big Is Your Baby?

By this week, your baby weighs almost 3¾ pounds (1.7kg). Crown-to-rump length is over 11½ inches (29cm), and total length is nearly 16¾ inches (42cm).

2.How Big Are You?

Measurement to the top of the uterus from the pubic symphysis is about 12¾ inches (32cm). Measuring from the bellybutton, it now measures almost 5 inches (12cm).

3. How Your Baby Is Growing and Developing

Twins? Triplets? More?

The rate of multiple births is going up—since 1980, the rate of twin births has increased 70%. Statistics show that close to 4% of all births in the United States are multiple births. If you’re expecting more than one baby, you’re not alone!

When talking about pregnancies of more than one baby, in most cases we refer to twins. The chance of a twin pregnancy is more likely than pregnancy with triplets, quadruplets or quintuplets (or even more!). However, we are seeing more triplet and higher-order births. A triplet birth is not very common; it happens about once in every 7000 deliveries. (Dr. Curtis has been fortunate to deliver two sets of triplets in his medical career.) Quadruplets are born once in every 725,000 births; quintuplets once in every 47 million births!

No matter how it happens, being pregnant with two or more babies can affect you in many ways. Your pregnancy will be different, and the adjustments you may need to make may be more wide-ranging. These changes may be necessary for your health and the health of your babies. Work closely with your healthcare provider to help make your pregnancy healthy and safe.

A multiple pregnancy occurs when a single egg divides after fertilization or when more than one egg is fertilized. Twin fetuses usually result (over 65% of the time) from the fertilization of two separate eggs; each baby has his or her own placenta and amniotic sac. These are called fraternal twins or dizygotic (two zygotes) twins. With fraternal twins, you can have a boy and a girl. Fraternal twins occur in 1 out of every 100 births. These rates vary for different races and areas of the world.

About 35% of the time, twins come from a single egg that divides into two similar structures. Each has the potential of developing into a separate individual. These are known as identical twins or monozygotic (one zygote) twins. Identical twins occur about once in every 250 births around the world.

Either or both processes may be involved when more than two fetuses are formed. What we mean by that is triplets may result from fertilization of one, two or three eggs, or quadruplets may result from fertilization of one, two, three or four eggs.

A twin pregnancy that results from fertility treatment most often results in fraternal twins. In some cases of higher-number fetuses, a pregnancy resulting from fertility treatment can result in fraternal and identical twins, when more than one egg is fertilized (fraternal twins) and, in addition, one or more of the eggs divides (identical twins).

The percentage of boys decreases slightly as the number of babies increases. In other words, as the number of babies a woman carries goes up, her chances of having more girls increases.

Special Issues for Identical Twins. With identical twins, division of the fertilized egg occurs between the first few days and about day 8. If division of the egg occurs after 8 days, the result can be twins that are connected, called conjoined twins. (Conjoined twins used to be called Siamese twins.) These babies may share important internal organs, such as the heart, lungs or liver. Fortunately this is a rare occurrence.

Identical twins may face some risks. There’s a 15% chance they will develop a serious problem called twin-to-twin transfusion syndrome. There is one placenta, and the babies’ blood vessels share the placenta. The problem occurs when one baby gets too much blood flow and the other too little. 

Dad Tip

Together with your partner, make a list of important telephone numbers and keep it with you. Include numbers for your work, your partner’s work, the hospital, the healthcare provider’s office, a back-up driver, baby-sitter or others. You may also want to make a list of numbers of people you want to call after the delivery of your baby. Take this list to the hospital with you.

There’s a chance that several different types of diseases may occur in identical twins during their lifetimes. This is less likely to happen with fraternal twins.

It may be important later in life for your children to know whether they were identical or fraternal because of health concerns. Before delivery, tell your healthcare provider you would like to have the placenta(s) examined (with a pathology exam) so you’ll know whether babies were identical or fraternal. It may be valuable information in the future. Even if there are two placentas, research shows it doesn’t mean twins are fraternal; nearly 35% of all identical twins have two placentas.

The Frequency of Multiples. The frequency of twins depends on the type of twins. Identical twins occur about once in every 250 births around the world. It doesn’t seem to be influenced by age, race, heredity, number of pregnancies or medications taken for infertility (fertility drugs).

The incidence of fraternal twins is influenced by race, heredity, mom’s age, the number of previous pregnancies and the use of fertility drugs and assisted-reproductive techniques. Twins occur in 1 out of every 100 pregnancies in white women compared to 1 out of every 79 pregnancies in black women. Certain areas of Africa have an incredibly high frequency of twins. In some places, twins occur once in every 20 births. Hispanic women also have a slightly higher number of twin births. The occurrence of twins among Asians is less common—about 1 in every 150 births. In Japan, only 6 sets of twins are born per 1000 births while in Nigeria that rate is over 7 times greater. In Nigeria, fraternal twins are born at a rate of 45 per 1000 births.

Heredity also plays a part. The incidence of twin births can run in families, on the mother’s side. In one study of fraternal twins, the chance of a female twin giving birth to a set of twins herself was about 1 in 58 births. The study also showed if a woman is the daughter of a twin, she has a higher chance of having twins. Another study reported 1 out of 24 (4%) of twins’ mothers was a twin, but only 1 out of 60 (1.7%; about the national average) of twins’ fathers was a twin.

If you’ve already given birth to a set of fraternal twins, your chance of having another set of twins quadruples! Other reasons for multiple fetuses include the use of fertility drugs, in-vitro fertilization, women having babies later in life, some women having more children, being very tall or obese, you recently discontinued oral contraception or taking large doses of folic acid.

Women having babies later in life accounts for nearly 35% of all multiple births. Age 30 seems to be the magic age beyond which the number of multiple births increases. Over 70% of all multiple births are to women over age 30. In the United States, the highest number of multiple births occurs in women over 40; the next highest group is women between the ages of 30 and 39.

The increase in multiple births among older women has been attributed to higher levels of gonadotropins. As a woman ages, gonadotropin increases, and she’s more likely to produce two or more eggs during one menstrual cycle. Most twin births in older women are fraternal twins.

Having more children (or pregnancies) can also result in more than one baby. This is true in all populations and may be related to the mother’s age and hormone changes.

Some families are just more “blessed” than others. In one case we know of personally, a woman had three single births. Her fourth pregnancy was twins, and her fifth pregnancy was triplets! She and her husband decided on another pregnancy; they were surprised (and probably relieved) when that pregnancy resulted in only one baby.

Discovering You’re Carrying More than One Baby. Diagnosis of twins was more difficult before ultrasound was available.

It is uncommon to discover twin pregnancies just by hearing two heartbeats. Many people believe when they hear only one heartbeat, there could be no possibility of twins. This may not be the case. Two rapid heartbeats may have a similar or almost identical rate, which could make it difficult to know there are two babies.

Measuring and examining your tummy during pregnancy is important. Usually a twin pregnancy is noted during the second trimester because you’re too big and growth seems too fast for one baby. Ultrasound is the best way to diagnose a multiple pregnancy.

Do Multiple Pregnancies Have More Problems? With a multiple pregnancy, the possibility of problems goes up. Possible problems include the following:

• increased risk of miscarriage

• fetal death or mortality

• birth defects

• low birthweight or growth restriction

• pre-eclampsia

• problems with the placenta

• maternal anemia

• maternal bleeding or hemorrhage

• problems with the umbilical cord, including entwinement or tangling of the babies’ umbilical cords

• too much or too little amniotic fluid

• abnormal fetal presentations, such as breech or transverse lie

• premature labor

• difficult delivery and Cesarean delivery


Ultrasound of twins shows two babies in the uterus.
If you look closely, you can see the two heads. The interpretive illustration shows how the babies are lying.

Birth defects are more common among identical twins than fraternal twins. The incidence of minor problems is twice as high as it is in a single pregnancy, and major defects are also more common.

One of the biggest problems with multiple pregnancies is premature delivery. As the number of babies increases, the length of gestation and the birthweight of each baby decreases, although this is not true in every case.

The average length of pregnancy for twins is about 37 weeks. For triplets it’s about 35 weeks. For every week the babies stay inside the uterus, their birth weights increase, along with the maturity of organs and systems.

It’s important to continue your pregnancy as long as possible; this may be achieved by bed rest. You may not be able to carry on with regular activities for the entire pregnancy. If your healthcare provider recommends bed rest, follow his or her advice.

Weight gain is important. You may be advised to gain more than the normal 25 to 35 pounds, depending on the number of babies you carry. With twins, if you were normal weight before pregnancy, you may be advised to gain 40 to 54 pounds. For overweight women, a weight gain between 31 and 50 pounds may be recommended; for obese women, a gain between 25 and 42 pounds may be recommended. If you’re expecting triplets, your weight gain may be between 50 and 60 pounds.

Some researchers believe use of a tocolytic agent (medication to stop labor), such as ritodrine, is critical in preventing premature delivery. These medicines are used to relax the uterus to keep you from going into premature labor.

Follow your healthcare provider’s instructions closely. Every day and every week you can keep the babies inside you are days or weeks you won’t have to visit them in an intensive-care nursery while they grow, develop and finish maturing.

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