Types of Twins and TTTS

We were warned from the very beginning that a twin pregnancy would get a lot more attention. More scans, midwife appointments, obstetrician appointments and more questions from people (are they identical, do twins run in your family, was it through IVF, would you prefer boys or girls, was it planned). So naturally I had to do my research. 

My research is from online and from conversations with my obstetrician or midwife. I’ve tried to make it as simple as possible but medical stuff is complicated! I’m not medically trained just curious.

According to the NHS website, statistically in the UK, multiple births have become more common than in my parents’ generation, due to fertility treatments and the fact women are having babies later. (I’m 32). In 1984 the chances of having twins was 1 in every 100 births. Today it’s more like 1 in 65. I thought the chances would be lower as I don’t know many people in my generation with twins, but then I don’t know anywhere near 65 people. 

Then there are the statistics (bear with me here) of the types of twins starting with one-third of all twins will be identical and two-thirds non-identical.

Non-identical twins (dizygotic) occur when two eggs are fertilised. The sex of the twins can either be of both sex or one of each. This type of twins is more common and would have two of everything; two eggs, two sperms, two placentas and two sacs.

Identical twins (monozygotic) happen when a single egg is fertilised. The egg then divides (at some point) in two, creating identical twins who share the same genes. Identical twins are always the same sex.

But…… then there are Monoamniotic-Monochorionic (Mono-Mono) and Monoamniotic-Diamniotic (Mono-Di) identical twins. (Still with me?)

Without including conjoined twins (which happen when the egg splits super, super late), Mono-Mono twins are one of the rarest as the embryo doesn’t split until the amniotic sac has already formed. These identical twins therefore share a sac and also the placenta. This type of pregnancy is high risk because of this and will need close supervision that the umbilical cords don’t get tangled up or because of twin to twin transfusion syndrome (TTTS).

Luckily, my egg decided to split earlier than that making these twins the other type, Mono-Di. They have separate sacs, but they share a placenta which makes this pregnancy mid risk for a few reasons one of which is TTTS. 

The benefit of paying for an early private scan at 6 weeks was being able to tell the midwife at my first appointment at 8 weeks that it was a twin pregnancy. It meant I could start being prescribed extra folic acid and ensure my future scan appointments were double slots so the sonographer could spend more time with us. It gave me more time to research the different types of twins too, the risks and how this pregnancy would differ to a singleton pregnancy (one baby).

I mentioned in my last blog post about how it pretty much felt like I was carrying a secret and not two babies during the first trimester. Apart from the private scans we paid for, not a lot happened. I didn’t feel or look pregnant, no one yet knew…… I just felt a lot of anxiety and impatience. Once into the second trimester, not much else happened either until a week before lock down. It was week 16 when we had an NHS growth scan and to hopefully find out the sex (we did – girls!). They confirmed again, as they did in all the previous scans that they were Mono-Di twins then asked us to go up to the maternity day care to plot the growth on the chart. After what felt like hours of waiting, they finally asked me through, took some more blood and told me that they had referred me to the Fetal Medicine Unit for two weeks’ time because twin 1 was measuring less than twin 2 and had been since the 12 week scan so they wanted to keep an eye on them. I had no idea what any of this meant, but thought if they were that concerned, they wouldn’t wait two weeks. 

Two weeks came around (Gestation 18 weeks 3 days) and lockdown had just started meaning partners were no longer allowed in the scan rooms or the day care unit in the hospital where I was for my FMU appointment. This of course was scary that I had to go alone, but we also felt lucky we were able to do a few scans together before lockdown started. I found out then that FMU appointments include a sonographer and an obstetrician at the same time so whilst the sonographer did the scan, the obstetrician would be there checking the results as they saw them. 

When I arrived, they explained a little more about why I was there which was the concern in the difference in the twin’s growth which was about a 9% discrepancy at the time. I’ve learnt from other twin mums online that it’s common for one twin to be a bit smaller than the other, but it’s a concern in both Mono-Di and Mono-Mono twins because they share the same placenta and are therefore at risk of twin-to-twin transfusion syndrome (TTTS). 

TTTS is a serious condition when there is an unbalance in blood supply meaning one twin would get too much blood and the other would get too little blood. It’s not great at all for either twin and without the right course of action would be fatal to both. There are a few signs to look out for; too much or too little urine in the twin’s bladder which leads to a decrease or excess amount of amniotic fluid, a thin hard to see membrane that separates the twins or signs of heart failure in one of the twins. If TTTS was present, it could involve draining the excess amniotic fluid or surgery. Both come with risks of breaking the waters and causing preterm labour.

18 weeks + 2 days

The signs of TTTS usually occur between 16-26 weeks, so I spent every other week back at the Fetal Medicine Unit to check for the signs as well as the usual measurements and checks. I felt extremely looked after and hopeful every time I left the appointment and they said there was no signs of TTTS. My last FMU appointment was at 28 weeks so am past the critical stage for TTTS (yay!) but they still want to see me every couple of weeks until my delivery week at week 36 as there is currently a 17% difference in growth which seems a lot, but without the signs of TTTS they were not concerned so neither am I.

As the weeks pass, I become less in denial at the fact I am pregnant with two babies! When I hit the 30-week mark last week, I decided to buy everything on my list that I needed ready for the two newborns. Up until then I hadn’t done any preparing apart from writing a list. I’ll be 31 weeks when this post is published meaning there’s only 5 weeks until the week in which they’ll be delivered. (Oh crap). My c section is currently being booked and should get my date in a few weeks’ time. It’s starting to get very real now. (Oh crap).

All that’s left now is to figure out how to actually look after a baby (well two of them). 

Next post I’ll be talking more about what it’s like to be pregnant during a pandemic and my choice to have a c section. 

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