It is estimated that around 5 % of the pregnant population (approximately 30 000 women per annum in the UK) are offered a choice of invasive prenatal diagnostic tests (most commonly amniocentesis or chorionic villus sampling). The type of diagnostic test available and offered is likely to vary depending upon the timing of any initial screening test that is performed.

Green Top guideline no 8, June 2010, Amniocentesis and CVS http://​www.​rcog.​org.​uk/​files/​rcog-corp/​GT8Amniocentesis​0111.​pdf

Amniocentesis in the third trimester is carried out for a number of indications, most commonly, for late karyotyping and detection of suspected fetal infection in prelabour preterm rupture of the membranes (PPROM). Serious complications are rare.

There is a suggestion that culture failure rates may be higher following third-trimester amniocentesis with a rate of 9.7 % reported in the series of O’Donoghue et al.

1. Green Top guideline no 8, June 2010, Amniocentesis and CVS http://​www.​rcog.​org.​uk/​files/​rcog-corp/​GT8Amniocentesis​0111.​pdf

2. O’Donoghue K, et al. Amniocentesis in the third trimester of pregnancy. Prenat Diagn. 2007;11:1000–4.

Decontamination of ultrasound probes between patients is variable, and there are practical difficulties in balancing the need for cleaning with prevention of degradation of the probe. Severe sepsis, including maternal death, has been reported following invasive prenatal procedures.

The level of risk cannot be quantified as case report literature does not provide denominator information, but the risk of severe sepsis is likely to be less than 1/1000 procedures. Infection can be caused by inadvertent puncture of the bowel, skin contaminants or organisms present on the ultrasound probe or gel.

Maternal RhD status should be available or obtained in every case. Prophylaxis with anti-D immunoglobulin must be offered following each procedure in line with national recommendations. Ultrasound gel may contain organisms, and many departments have mechanisms to minimise the risks including the use of sterile ultrasound gel when performing invasive procedures.

In case of maternal HIV infection, the risk of transmission should be discussed and consideration given to starting antiretroviral therapy to reduce the viral load prior to the procedure. Review viral load and treatment regimens prior to invasive prenatal testing in women with HIV and consider delaying the procedure until there is no detectable viral load if the woman is already on treatment.

Consider antiretroviral therapy prior to prenatal invasive procedures in women not yet on treatment for HIV.

1. Green Top guideline no 8, June 2010, Amniocentesis and CVS http://​www.​rcog.​org.​uk/​files/​rcog-corp/​GT8Amniocentesis​0111.​pdf

2. Backhouse S. Establishing a protocol for the cleaning and sterilisation/disinfection of ultrasound transducers. Br Med Ultrasound Soc Bull. 2003;11:37–9.

Ultrasound diagnosis of chorionicity was based on demonstration of the ‘lambda’ or ‘twin peak’ sign (dichorionic) or ‘T-sign’ (monochorionic) at the membrane–placenta interface.

When the gender of the twins is dicscordant—it indicates they are dizygotic (arising from different zygotes) and hence dichorionic (two placentae)

Two separate placental masses are obviously ‘dichorionic’. The option ‘bicornuate uterus’ is a simple distractor.

Green-Top guideline No. 51, December 2008, Management of monochorionic twin pregnancy http://​www.​rcog.​org.​uk/​files/​rcog-corp/​uploadedfiles/​T51ManagementMon​ochorionicTwinPr​egnancy2008a.​pdf

Chorionicity in multifetal pregnancy is established by assessment of placental signs like the ‘twin peak’ sign (lambda sign) indicating dichorionicity and ‘T-sign’ indicating monochorionicity. Chorionicity is better assessed by ultrasound before 14 weeks than after 14 weeks. The optimal time for chorionicity determination is 11–14 weeks as very early in pregnancy, while for 5–6 weeks, the placental masses are not well developed.

Green-Top guideline No. 51, December 2008, Management of monochorionic twin pregnancy http://​www.​rcog.​org.​uk/​files/​rcog-corp/​uploadedfiles/​T51ManagementMon​ochorionicTwinPr​egnancy2008a.​pdf

Twin to twin transfusion is a complication of monochorionic pregnancies. The diagnosis of TTTS is based on ultrasound criteria: the presence of a single placental mass, concordant gender, oligohydramnios with maximum vertical pocket [MVP] less than 2 cm in one sac and polyhydramnios in other sac (MVP ≥ 8 cm) (some would say ≥ 8 cm at ≤ 20 weeks and ≥ 10 cm over 20 weeks) 12, discordant bladder appearances, severe TTTS, haemodynamic and cardiac compromise, severe TTTS.

The option B—discordant gender—represents dichorionicity and hence is not a criteria to diagnose TTTS.

Green-Top guideline No. 51, December 2008, Management of monochorionic twin pregnancy http://​www.​rcog.​org.​uk/​files/​rcog-corp/​uploadedfiles/​ T51ManagementMonochorionicTwinPregnancy2008a.pdf

TTTS complicates 10–15 % of MC pregnancies; the placentas are more likely to have unidirectional artery–vein anastomoses and less likely to have bidirectional artery–artery anastomoses.

TTTS is unique to monochorionic placentae.

Do not monitor for feto-fetal transfusion syndrome in the first trimester. Start diagnostic monitoring with ultrasound for feto-fetal transfusion syndrome (including to identify membrane folding) from 16 weeks. Repeat monitoring fortnightly until 24 weeks.

Ultrasound examinations between 16 and 24 weeks focus primarily on detection of TTTS. After 24 weeks, when first presentation of TTTS is uncommon, the main purpose is to detect fetal growth restriction, which may be concordant or discordant.

1. NICE clinical guidelines 129, multiple pregnancy: The management of twin and triplet pregnancies in the antenatal period issued: September 2011 guidance.nice.org.uk/cg129 http://​www.​nice.​org.​uk/​nicemedia/​live/​13571/​56422/​56422.​pdf

2. Green-Top guideline No. 51, December 2008, Management of monochorionic twin pregnancy http://​www.​rcog.​org.​uk/​files/​rcog-corp/​uploadedfiles/​T51ManagementMon​ochorionicTwinPr​egnancy2008a.​pdf

With regard to noninvasive prenatal testing for fetal aneuploidies, published data indicate extremely good results for trisomy 21 and trisomy 18 prediction when sequencing is successful. For trisomy 21 and 18, the detection rates in large series using different technologies have reported (after successful sequencing) sensitivity and specificity close to 100 %. However, there are few possible sources of error in this technology, and these include:

Maternal heart disease and placenta previa are simple distractors.

http://​www.​rcog.​org.​uk/​files/​rcog-corp/​SIP_​15_​04032014.​pdf

Multifetal pregnancy is a condition with two or more foetuses in the uterus. The three important features of multifetal pregnancy are ‘zygosity’, ‘chorionicity’and amnionicity. Each zygote can form its own placenta (chorion) and sac (amnion).

However, if a zygote divides with 1–3 days of fetulisation, it can form two separate placenta and two separate sacs—dichorionic diamniotic. Some other possibilities are as follows:

As a general rule, the number of sacs should be equal to or more than the number of placentae; hence, combinations like dichorionic, monoamniotic are not physiologically possible.

Dewhurst’s textbook of obstetrics and gynaecology, 7th edn, edited by Keith Edmonds, Chapter on Multiple pregnancy (p. 166–76)

A medical pedigree is a graphic presentation of a family’s health history and genetic relationships.

Example of a pedigree drawing.

Each family member is represented by a square (male) or a circle (female). Individuals with unspecified gender are represented by ‘diamonds’. They are connected to each other by lines (relationship lines). Vertical lines connect generations. Horizontal lines are ‘mating lines’ and consanguineous mating is represented by two horizontal lines. Usually 3 generations are depicted in a standard pedigree, and this helps in establishing the pattern of inheritance and assists in identifying persons at risk of an inherited disorder.

Benett et al. Standardized human pedigree nomenclature: Update and assessment of the recommendations of the national society of genetic counselors. J Genet Counsel. 2008;17:424–33.

Link to article: http://​geneticcounselin​gtoolkit.​com/​cases/​pedigree

Change in AC or EFW may improve the prediction of wasting at birth (neonatal morphometric indicators) and adverse perinatal outcome suggestive of FGR.

When using two measurements of AC or EFW to estimate growth velocity, they should be at least 3 weeks apart to minimise false-positive rates for diagnosing FGR. More frequent measurements of fetal size may be appropriate where birth weight prediction is relevant outside of the context of diagnosing SGA/FGR.

SGA fetus – RCOG Green Top guideline No. 31. Feb 2013. Minor revisions Jan 2014 http://​www.​rcog.​org.​uk/​files/​rcog-corp/​GTG31SGA23012013​.​pdf

Achondroplasia has an autosomal-dominant (AD) mode of inheritance, while others have an autosomal recessive (AR) mode of inheritance. Hence, the recurrence risk for achondroplasia is 50 %, while for the conditions inherited in AR manner, it is 25 %.

Davidson’s principles and practice of medicine, 22nd edn. Chapter 3: Molecular and genetic factors in disease.

Monochorionicity confers major increases in perinatal morbidity and mortality when compared to dichorionic gestations. Chorionicity may be accurately determined by ultrasound between 10 and 14 weeks gestation. A number of methods are widely used, including the presence of the lambda or ‘twin peak sign’ for dichorionicity and the ‘T-sign’ for monochorionicity. In addition, the thickness of the intertwin membrane may be determined—a membrane thickness of less than 2 mm is suggestive of monochorionicity. Other indicators in the second trimester may be the presence of two separate placental masses or discordant fetal sex. If it is difficult to determine chorionicity, even after senior referral, the pregnancy should be managed as monochorionic until proven otherwise.

Bonney E, Rathod M, Cohen K, Ferriman E. Twin pregnancy. Obstet Gynaecol Reprod Med. 2013;23:6.

Congenital adrenal hyperplasia (CAH) is a spectrum of autosomal recessive conditions in which there is a mutation in the coding of one of the many enzymes responsible for cortisol synthesis from cholesterol in the adrenal gland, resulting in defective enzyme production. About 1 in 60 of the Northern Europe population is a carrier of CAH. All affected families—either homozygous mothers planning a pregnancy or families with a child previously affected—should be offered genetic counselling. The family with a previously affected child has a 1 in 4 chance of having a subsequently affected offspring and a 1 in 8 chance that the child will be a virilised female.

Girling J, Cotzias C. Thyroid and other endocrine disorders in pregnancy. Obstet Gynaecol Reprod Med. 17(12):349–55.

Dexamethasone readily crosses the placenta. It should be started prior to 7 weeks gestation (when the external genitalia begin to differentiate) to suppress fetal ACTH excretion and androgen overproduction. This dexamethasone regimen, 1–1.5 mg daily, reduces the need for corrective surgery for virilisation but is not always successful. Early fetal sexing by identification of cell-free DNA in maternal blood identifies a male fetus and means that dexamethasone can be stopped appropriately early. Alternatively a CVS at 11 weeks can be used to determine whether the female fetus is affected or not and therefore helps to time the cessation of dexamethasone therapy.