Two Placentas, One Baby – Incredible Facts

Pregnancy is full of endlessly fascinating facts.

Your heart actually grows bigger in response to more blood being pumped around your body.

Babies can actually cry in the womb (with no sound of course).

And it’s possible to have two placentas but only one baby in there.

Really! Let’s find out what makes two placentas and one baby a possibility during pregnancy.

Placentas what are they?

The placenta is an amazingly complex organ that develops inside the uterus as your pregnancy progresses. It is the only organ of the body that grows within another organ.

It has many functions, and is essential for the growth and development of your baby.

Your baby needs a constant supply of nutrients, oxygen, water and antibodies (along with many other things). All of these are delivered directly to your baby via the placenta and umbilical cord.

The placenta works as a two-way system. Waste products, such as carbon dioxide, are also transferred from the baby, back through the umbilical cord to the placenta, to be processed by the maternal system. 

You can read more fascinating placenta facts here. 

Can you have two placentas?

In the days and weeks that follow conception, the swiftly multiplying cells become your baby and placenta. If you are carrying one baby (singleton pregnancy), then only one placenta will develop.

It’s possible for more than one placenta to form – for example, if you are pregnant with twins or triplets. This is the most common reason to have two placentas in pregnancy. 

Multiple pregnancies are becoming more common due to the increase in fertility treatments and because women are having babies later in life.

According to the NHS, one in every 65 births in the UK will be twins, triplets or more. The Centers for Disease Control and Prevention states twin births have nearly doubled over the last 40 years in the US, and triplets and higher order births have quadrupled. 

Twins

You will generally find out if you are having twins at your first trimester or early pregnancy scan, at around 8-14 weeks. The scan will usually be able to tell you whether your babies share a placenta or whether there are two placentas. 

Types of twins

There are three types of twins:

• Identical or monozygotic twins. This is when one egg is fertilised and then splits into two (or more) to create two identical babies. Identical twins carry the same genetic material, and are always the same sex. One third of all twins will be identical.
• Non identical or dizygotic (fraternal) twins. In this case, two separate eggs are fertilised. Non identical twins are no more likely to be alike than any other siblings. Non identical twins can either be both girls, both boys, or one of each. Two thirds of twins are non identical. 
• Superfecundation. This happens when two (or more) eggs from the same cycle are fertilised by sperm from two separate acts of intercourse. This is extremely rare, and results in a variant of non identical twins. Twins of this nature can either be homopaternal (both with the same biological father), or heteropaternal (with separate biological fathers), if intercourse with more than one partner has taken place within the same menstrual cycle. 

Do identical twins have one or two placentas?

Identical twins may share the same placenta, or can have two placentas that are separate. This depends on how early in the developmental process the single fertilised egg split into two.

Having non identical twins is almost like having two separate pregnancies occurring at the same time. The twins will have their own individual placentas. If the two fertilised eggs implant very close to each other in the uterus, however, it can result in the two placentas fusing together. Two fused placentas might look like one. 

If you’re pregnant with twins, be sure to check out our other articles about twins.

Vanishing twin syndrome

This phenomenon, also known as ‘missing twin’ syndrome, occurs when there’s a spontaneous loss (miscarriage) of a developing baby in a multiple pregnancy.

Two gestational sacs are detected on an early ultrasound but, later on, there is only one heartbeat, and the second sac has disappeared.

The ‘vanishing twin’ can sometimes be seen on a later ultrasound as a blighted ovum, alongside a normally developing baby. A blighted ovum is a fertilised egg that attaches to the uterine wall, but doesn’t develop into an embryo. 

Sometimes the two placentas, including that of the ‘vanishing twin’, can be detected when a mother has gone on to grow a healthy singleton baby. This can be seen on ultrasound, or is noted with the birth of the baby upon examination of the placenta.

A mother might never have been aware there was ever a second baby, or two placentas. Vanishing twin syndrome occurs in 10-40% of multiple pregnancies. Experts agree it’s hard to determine exactly how often it occurs, however, as not all women have a first trimester ultrasound. 

What does a double placenta mean?

There are various placental malformations which might have the appearance of two placentas, even though you are pregnant with a single baby. 

These placental differences include:

• Bipartite placenta (bilobed, bilobate or placenta duplex). The placenta is divided into two fairly equally sized lobes, separated by a membrane. This can sometimes be detected on ultrasound and can be mistaken for two placentas. It occurs in 2-8% of placentas. The umbilical cord might be inserted into either lobe, or between the lobes. There doesn’t appear to be any increased risk of fetal abnormalities with a bipartite placenta. However, it is associated with an increased risk of first trimester bleeding, polyhydramnios (too much amniotic fluid around the baby), abruption, and retained placenta. 
• Tripartite placenta (multipartite or multilobed placenta). This is the same as a bipartite placenta, but with three similar sized or equal lobes. 
• Succenturiate lobe (‘accessory’ or supernumerary placenta). One or more ‘accessory lobes’ develop in the membrane, separate from the main placenta. The lobes are linked by a vessel to the main placenta. A succenturiate lobe is a smaller variant of the bi/tripartite placenta. This occurs in 5% of placentas. According to research, increased maternal age and IVF treatments increase the incidence of succenturiate lobes. The succenturiate placenta can cause complications, including an increased risk of having a retained placenta, leading to excessive bleeding or postpartum hemorrhage. 

Most serious placental abnormalities can now be detected by ultrasound. They’re usually picked up at your mid trimester scan (also known as the anomaly scan), at 18-20 weeks.

As with any modern technology, scans aren’t perfect, and are subject to human error, so occasionally things can go undetected. Many placental malformations are only discovered during or just after birth. 

Please speak with your midwife or doctor straight away if you are known to have a placental variation, or experience any of the following symptoms: 

• Abdominal pain
• Vaginal bleeding
• Uterine tenderness
• Pain in between contractions during labour.

While two placentas and only one baby isn’t impossible, it’s not usually something that happens often.

If you’re not sure what it means for you, make sure you talk to your healthcare provider.